The reason why the engine came loose was that the metal in the screwheads became "exhausted" of all the vibrations from the engine. It was a very unrealistic senario because all of the srewheads came lose all at the same time.
not unrealistic at all, that happens surprisingly often in ready to fly planes. not always the screws, sometimes the firwall itself... I ALMOST had it happen to my 25cc gasser. I brought it back to refuel and put another flight on, when I noticed the engine wasn't quite as tight as it should have been. turns out most of the firewall had ripped free, and the rest was barely hanging on.
moral of the story is do NOTHING in front of the engine beside start the thing. don't ever reach over to remove the glow plug, that is probably the worst thing to do ever. You have to end up behind it anyway, just wait a couple more seconds to walk around and then remove it.