Garfield touched upon the mechanical forces involved with how a sudden stoppage of the crankshaft will cause the already spinning mass of the prop to loosen itself from the locked up engine. A mechanical bind or seizure inside the engine will do the same.
I have seen a tapered collet style drive washer fail [ST .40] because the owner went berserk with prop tightness, it's usually pretty intuitive when enough is enough. A 6 inch long box end wrench and enough torque to open a jar of peanut butter should be enough.