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.

Love it and not a bad description either. If we worked to standard engineering practises then a 1/4" nut would be tightened to about 80 inch pounds (near enough 7 foot pounds) and a 5/16" nut to about 120 inch pounds (10 foot pounds). That's only a 14 or 20 pound force on the end of a 6" spanner (wrench, if you like).

Most prop drivers are machined from bar stock but the ST prop drivers are cast and hollow inside. They're designed to handle the forces involved but don't allow for the occasional gorilla that buys an ST .

the prop was being installed properly, turns out the engine was flooded. Thanks for the help guys.

love those plastic spinners on big 4 strokers....Slight kick back and im looking for the drivers....

Come on down alloy spinner......