This was unfortunate, it's hard to see what happened but it may have been his back wheel catching the rock and bouncing him off. We think his arm ran into my chain and sliced it open, which is ironic because he's got a bike chain tattoo which was cut now. It was a fairly nasty cut on his right arm, taking a big chunk of skin off.
https://www.youtube.com/watch?v=HvgQzuFpGts