ORIGINAL: HighPlains

OK, I've read all the responses and here is the straight dope.

Throwing a prop or having the prop get loose on a two cycle engine is due to the engine back-firing while starting. This is caused by having too much fuel in the engine. It is so common to over-prime and then hit the engine with the starter. The inertia of the prop, starter, and spinner runs into the back-fire of the engine. This results in the prop getting loose or being thrown.