You mean the backplate, prop, and spinner cone came loose?

You've probably flooded your engine. When you hit it with the starter the engine won't turn over, or offers much higher than normal resistance, due the large amount of fuel hanging around in there. Result: the starter spins the prop off.

Try less prime. Better yet, have a go at hand starting.

To clear the fuel, take the glow plug off and spin the engine with the starter. Be warned: fuel will spray out of the glow plug hole when you do that.