There are quite a few reasons why a horse may jig or buck. Here are a few.
(1.) Sore back. Poor saddle fit, in sufficient or worn pad, vertebrae out of alignment, etc. (Reference #16).
(2.) Girth sores, insect bites or a minor injury where girth or saddle is positioned (or sensitivity from same).
(3.) Mouth sensitivity. Includes (but not limited to) mouth/tongue/lip injury or sores, hot spot from bit or sharp spot on bit, abusive/improper use of the reins (commonly referred to as 'bad hands') ‘tongue-over bit or a horse that is unaccustomed to the restricted breathing/choking sensation of the bit. (Reference #16.)
(4.) Eyesight. Diminished visibility due to old age, temporary infection or injury that is not immediately obvious, which would increase apprehension/fear due to the horse's diminished ability to survive insofar as detecting a predator attack.
(5.) Too little regular association/interaction, exercise and/or no habit or pattern established of doing even limited riding on a fairly regular basis to imprint a positive Life Pattern.
(6.) Too much and/or too ‘hot/rich’ of a supplemental feed. (Excessive energy, etc.)
(7.) Internal ulcers, illness/trauma, (whether chronic or acute) etc, that the horse tolerates under less stressful, normal living conditions but is forced to exhibit reactive discomfort/displeasure when engaged in mounted activities.
(8.) Growth spurt causing a young horse to test and possibly attempt to reestablish himself to a higher herd rank than his rider, (also know as the ‘terrible Twos and Threes’ – comparable to a human teenager. Unless in a conscientious ‘loading program,’ a two or three year old should not be ridden.
(9.) Emergent Emotional Intelligence/Maturity. While two and three year olds may look physically fit to carry a rider and tack, the equine bone structure does not mature until it is six or seven years old. Their back is the last part of his bone structure to mature. This is directly proportionate to his emotional maturity and would be comparable to expecting the average four-year-old human to sit attentively through an entire opera without once squirming or squiggling in impatience and/or distractive inattention. (If this is the case, you are riding a horse that is not physically or emotionally mature enough to be ridden.)
(10.) Abnormal need for ascension in herd rank (genotype). This is exemplified when a horse of small stature and low herd rank is constantly seen with injuries caused by his continuous, insistent challenging horses of higher herd rank that are forced to wreak physical punishment for CONTINUALLY challenging them.
(11.) Abnormal aggressiveness (genotype) as displayed by an Alpha's constant physical attacks on other horses of lower herd rank for seemingly no apparent reason. While genetic in origin, it may also be aggravated by a lack of confidence in maintaining present herd rank (similar to the proverbial grade school bully). This abnormal aggressiveness may also be due to a complete lack of formative, early life
socio-cultural learning facilitated by our present day care and management practices of ‘early weaning.’
12.) The horse lacks self-confidence in the rider and feels that his herd rank, position and/or very survival is threatened by submission to the rider. (Reference #18 & #21.)
(13.) Bipolar disorder, (and/or other possible neurological disease/trauma.)
(14.) A mare's overreaction and abnormal sensitivity to estrous, (severe PMS). (Also pain caused by Ovarian cysts.)
(15.) An adverse drug reaction, (oral or subcutaneous/intravenous injection) consumption of toxic plants or contaminated hay/feed may cause chronic and/or acute pain and/or sudden mood changes.
(16.) Previous life experiences associated with a negative stimuli implanting a fear imprint on the Amygdala. Either abusive handling, abusive mounted activities and/or a mild/severe injury in what the horse deemed was a life-threatening situation. Associative situational circumstance activates an abnormal oppositional or fear/flight/fight response (similar to Post Traumatic Stress Disorder in humans.) This can be triggered by a situational environmental stimuli as well as a physical touch to a specific part of the body, a specific sound, or a specific scent.
(17.) Positive Response trigger/imprint is not deep enough to control and/or calm horse with a verbal 'Calm Down cue' when the horse feels the hyper-reactive excitation of a sudden severe survival/stress situation (cortical override of a fear/flight reaction to a negative stimuli induced by an Amygdala fear imprint).
(18.) Insufficient relationship/trust factors. Suppressive imprint is inadequate to instill cortical override and nullify previous fear imprints and/or separation anxiety disorder. Inadequate trust factors and nonreciprocal communication levels result in a confrontational relationship versus a harmonious partnership. Complete absence of a symbiotic Peer Attachment relationship.
(19.) Emotional trauma (oppositional defiance disorder) caused by excessive solitary confinement (stalling) and/or isolation due to a lack of interaction with other horses and/or natural freedom of movement.
(20.) Physiological, (musculo-skeletal) circulatory deficiencies resulting from solitary confinement, (excessive stalling).
(21.) PTSD / fear imprint triggered by association to the specific olfactory or visual stimuli (cologne/deodorant) or apparel/accessory such as a particular hat, coat, etc, (may also be gender specific).
(22.) Rider induced lameness, soreness or discomfort caused by a rider that is unfamiliar with the biomechanics of the horse's body in movement carrying a rider. (Reference #16.) Inexperienced or apprehensive/fearful rider that has not acquired the necessary balance, coordination, independent seat and confidence needed. While rider induced lameness is a possible cause of varying degrees of physical trauma/disability/stress, rider-induced stress caused by the novice, inexperienced rider’s own apprehensive heart rate, emotional state and anticipatory fear are transmitted to and easily detected by their horse. (Which results in a ‘catch 22 self-fulfilling prophecy of doom.’)
Ref: Linda Keeling, PhD, and colleagues at the Swedish University of Agricultural Sciences and Professor Ellen Gehrke, Alliant International University and the Institute of HeartMath.
(23.) Fear/pain imprint reaction actuated by losing tolerance to the emotional pressure of a specific situational environment (being ridden-and/or ridden specific places). This may have been caused by the present rider on his back or a previous rider that exhibited unfair treatment and/or physical punishment, ( a form of PTSD). An example would be being whipped to cross a creek or small, shallow water body of water.
(24.) Teeth: #1. TMJ misalignment due to improper/inadequate 'floating’ and bite alignment resulting in limited biomechanical function of the temporomandibular joint, (TMJ). This sequentially affects ALL parts of the body from the head and neck to the back, legs and feet). Ref: Spencer La Flure
(25.) Diminished, or diminishing eyesight, (partial blindness in either or both eyes) will elicit avoidance and oppositional defiance when a horse is forced to carry a rider away from their normal living environment, (comparable to asking a human to run blindly through thick fog at breakneck speed in a graveyard or ‘bad part of the city’).
(26.) Bone spurs, wolf teeth, blind wolf teeth can also cause a great deal of pain, (especially when bitted) which in turn elicits aversion, avoidance and oppositional defiance behavior. There is no reason for a horse to have a bit in their mouth, period.
(26.) Hoof problems. Pain from overgrown bar, high heels, contraction, deteriorated frogs, thrush and abscesses are common and often masked by shoeing until it is so serious as to cause visible lameness. Horses adapt by a shifting resting posture to alleviate pain and strain on ligaments and tendons, pain in the joints and muscles of the shoulder, neck, back, hocks, hips, etc. and by moving with shortened strides exhibiting toe first or flat footed landings.
(27.) *I have also heard that the girth can press on the Vagus nerve of some horses causing an irregular heartbeat that results in extreme cinchiness’ and or bucking when first mounted. The Vagus nerve is the tenth cranial nerve. It is a mixed sensory and motor nerve.
It is my understanding that the neurological system is divided into central and peripheral areas - the brain and spinal cord make up the Central Nervous System (CNS). There are 12 cranial nerves that originate in the brainstem to innervate the organs of sight, smell, and hearing, the muscles of swallowing and mastication, the tongue, sensation of the face, and use of the eye and facial muscles. Abnormalities of these nerves will produce changes in head carriage, balance, eye position, ear and eyelid tone and position, vision, smell, hearing, and problems prehending, chewing, and swallowing food. The 10th cranial nerve, the Vagus nerve, also affects cardiac function, respiratory function, and GI motility.
Given the horse's incapability to communicate through the use of traditional training formats, he has little choice but to disobey by jigging, spooking and/or bucking when, in HIS judgment, due to fear, apprehension, pain, chronic discomfort and/or frustration as HE thinks the situation warrants.
*Any of the preceding (or any combination thereof) could very well result in the ‘spooking at nothing’ of an obviously familiar object to ‘jigging’ or actual bucking in an attempt to dislodge the rider and a 'Jekyll and Hyde' personality.
This is traditionally viewed as ‘bad behavior’ when in fact it is a complete lack of insight, knowledge and understanding on the part of the rider.