Brakes. Step through the slow-mo replay, you can see the chain's flopping about all through the crash. Plus, if it had jammed, it wouldn't have un-jammed by itself.
Why is it the R6 owners? I had a kid come take a basic class; rode his R6 up after lunch, asked what I thought. (I thougth if you were clever, you would have got endorsed before the $720 ticket / court ordered class... but I kept that thought in my head.)
I reached out with thumb and finger and plucked the chain off the rear sprocket and said "I think you should fix that..." He had no idea that chains need any sort of maintenance or adjusting.