Excessive speed (RPM's) is one reason, another reason is prop tip flutter.

I had a very dramatic (and cool) demonstration of this on my 100cc engine. I put a 24/12 3 blader on it to try and when applying full power on the ground, you could actually watch the prop tips deflect about an inch forwards!

Prop flutter needless to say is not healthy for anything involved or effecient at all, but it sure sounds cool