Bulls, being prey animals, do not like having something on their backs. In the wild it might be a lion or some predator rather than a relatively harmless human.
Bulls employ a number of techniques to buck off the rider which are called things such as belly rolls and spins. The rider attempts to stay on the bull for 8 seconds without touching the bull with his free hand. The rider that manages to complete a ride is scored on a scale of 0-50 and the bull is also scored on a scale of 0-50. Scores in the 80s are very good and in the 90s are exceptional.
See also: rodeo