Newton's First Law

"An object in motion will remain in motion.  An object at rest will remain at rest unless acted upon by an outside force."

How this pertains to Bowling

Before the ball smashes into them, the pins are stationary.  After the ball hits them, the ball acts as an outside force to the pins and causes them to start moving.  True to the Law the only thing that could stop the ball is the back wall of the lane.