It's amazing how bolts will just drop off at times, even though this is usually rare. Vibration will do it over time, usually a very long time, although I have seen exhausts un-wind before my eyes due to vibration, even though they were put on very tight.
I also had all four bolts come off on a Rover mower disc one time, which has me baffled to this day. I had only just put them back on and had tightened each bolt fully. I had a mulcher plug fitted in the rear and was mowing some very heavy, soft clover, which contained a lot of moisture. I went three rounds of the lawn and next thing the entire blade disc dropped off and almost burrowed itself into the dirt underneath. I could not believe it. Every bolt had unwound until they had fallen off. I've never had that happen before and it has never happened since. I put it down to Gremlins.
I am way off topic here, but just thought I'd throw in a couple of similar "dropping off" moments, if for no other reason than sympathy. :-)
