That's a cracking picture though...over the bar crashes like that always feel like they happen in slow mo. You know what's going on buy are powerless to do anything about it.
I doubt it, not a crash like that. Only way that would really happen was if it jumped and he grabbed loads of front brake which isnt really the chains fault. Its at the rear so shouldnt cause you to flip over the bars.
The chain can jump on any setup if setup incorrectly.