Momentum is conserved when no external forces act on a system. If Bronco alone is our system, then momentum is not conserved when he falls and speeds up. We must consider the whole Earth and Bronco as our larger system. Then, the change of momentum of Bronco (downward impulse) is balanced by an equal and opposite change of momentum of the Earth (an upward impulse that results from Bronco pulling up on the Earth). The momentum of the larger system is conserved, and the law is not violated.