I've had a wood prop come loose while hand starting because the wood had compressed slightly but if the engine had fired then that tries to tighten the nut. Any time an engine is running properly it's trying to keep the nut tight on every firing impulse so to have one come loose and throw the prop means something is wrong (unless you've got a reverse rotation engine ). A 2 stroke usually won't backfire like a 4 stroke does to kick it loose but if you're using too much nitro or too hot a plug then the ignition is too far advanced and this can loosen the nut.