There are a few common causes for throwing the prop.

Running the engine lean in the air. The lean run can cause a bit of early detonation which losens the prop nut. Older 4-strokes had this happen all the time because they were easier to accidentaly set lean, and the lower RPM made it easier to backfire, but 2-strokes can do it too. Richen up your mixture a couple of clicks and it should go away.

If the engine is flooded and you use an electic starter, I've seen the starter cause a backfire that throws the prop.

Wooden props sometimes compress with time. You put the prop on nice and tight, and fly, but the wood slowly compresses, causing the prop nut to become loose. If you are using wood props, try something non-wood. Or be sure to check the nut once in a while.