I'd say you are more lucky that nobody has been hit by it. As bigchap mentioned, you don't have the prop nut tight enough. Use a 6" wrench and get it tight. There should be some type of jam nut on the prop shaft in addition to the primary nut. Backfiring is usually a sign that the engine isn't getting enough fuel. Do you have the glow driver on constant or does it disengage as the throttle increases? I suppose it's possible that the right circumstances could be igniting the fuel mixture too soon and causing the backfires. Try flying without the on-board glow but definitely get the prop nut tighter.