Bucking

Bucking is a movement performed by a horse or bull in which the animal lowers its head and raises its hindquarters into the air, usually while kicking out with the hind legs. If powerful, it may unseat the rider enough to fall off.

Bucking, though a potentially dangerous disobedience when under saddle, is a natural aspect of horse behavior. It developed in the wild for the purpose of protection from feline predators such as mountain lions, who would attack horses by dropping onto their backs from above. The process of kicking out with both hind legs, another defense mechanism for the horse, also results in a mild bucking movement. Thus, for a human to safely ride a horse, the horse has to be desensitized to the presence of something on its back and also learn not to kick out with both hind legs while under saddle. Nonetheless, because the instinct is always there, bucking can still occur for a number of reasons:

Ordinary riders need to learn to ride out and correct a simple buck or two, because it is a relatively common form of disobedience. Further, at times, movement akin to bucking is actually required of a horse: Horses that are jumping over an obstacle actually are using almost the same action as bucking when launching themselves into the air, it is simply carried out with advanced planning over a higher and wider distance. The classical dressage movement known as the Capriole is also very similar to the low buck done by a horse when it kicks out with both hind legs.

Bucking, especially if triggered by fear, pain or excitement, is generally a minor disobedience, unless it is strong enough to unseat the rider, at which point it is a dangerous act. If bucking is a premeditated act of the horse and becomes an undesired habit (such as when a horse learns to buck off a rider so as to no longer have to work), then the horse must be re-schooled by a professional trainer. There have been Olympians who have had to send their horses for re-training by a specialist.

It is important to address the problem of the bucking immediately. Even with good cause, it is a potentially dangerous disobedience that cannot be encouraged or allowed to continue. However, a rider does need to be sure that it is not poor riding that is causing confusion, or a result of poorly-fitting tack that is causing the horse pain. The horse's turn-out schedule should also be assessed, as extra turn-out will give a horse to release his extra energy before the rider gets on. In certain cases (such as a show, when horses are unable to be turned-out for extended periods), longeing the horses for a brief period can help calm him enough so that the rider can get on.

...
Wikipedia