Either the crank was not on tight enough to begin with, or it loosened up and then rounded out. Is it a square-taper bottom bracket? After i put new cranks on, I check them after a ride to make sure they are still tight. The problem may also lie in that the bottom bracket's square-taper has been rounded out. This would cause the cranks to wear, and eventually become loose. Check that first. Maybe take both crank arms off and see if the bottom bracket looks different on the loose side.