ORIGINAL: Dusty1
I've seen this a couple times before. People may do a range check prior to flying but they don't always think to check prop nuts. So it's quite possible that his spinner nut was loose prior to his taking off.
Yes, the nut had to be a bit loose or a backfire wouldn't have spun it off. but without a backfire.. it COULDN"T spin off in flight. The normal rotation of the engine acts to tighten the nut. (which is why electric starters can so often spin the nut right off.)