Due to the normal crankshaft rotation, the prop nut is constantly being tightened with engine running---that's if everything is working right!---prop nuts loosen when the engine decelerates rapidly while the prop inertia breaks the nut loose---four stroke engines decelerate rapidly just from quick throttle shut-down---they also decelerate rapidly due to lean high speed fuel mixtures which cause detonation, a form of extremely advanced ignition timing that can cause an engine to stop abruptly---yours stopped, & the prop nuts were loose right?---if it looks like a gorilla & its eating bananas, its probably a gorilla---get the proper nuts, install them properly & set carb. high speed mixture correctly---hope this helps.......