I've seen several of those as well. Like you, I don't know what's causing it either, and have no idea what's going on with the triggers when it does happen.The sig 320 firing without input has been demonstrated many, many times in the past few years. I’ve seen video of a police officer from my state searching a car and his holstered sig firing. What’s causing it, I don’t know, but you don’t see or hear of glocks doing it.
Maybe a better way to say it is that it's not being replicated and not being explained in these same pistols after the fact. A partially pulled trigger could explain it. I agree though, it certainly seems to be a 320 thing when it's happening.