Horses buck in rodeos primarily due to a combination of natural instinct, physical discomfort or pain, and environmental stimuli. The bucking behavior evolved as a natural defense mechanism to throw off predators, and in rodeos, it is often triggered by the presence of a rider and the pressure of equipment like the flank strap — a strap around the horse's flank that encourages bucking by causing an unfamiliar sensation. Additionally, some rodeo horses are selectively bred and trained for their athleticism and natural tendency to buck, making it a performance behavior in this context. Pain-related causes, such as conditions like "kissing spines" (where vertebrae crowd together painfully), can also provoke bucking as a reaction. Fear, previous negative experiences, and learned behavior can further contribute to this action, as horses may buck to try to remove the cause of their discomfort or rider. Overall, bucking in rodeos is a complex interplay of instinct, physical condition, training, and environment.