I don't see how the chain locked the rear wheel so I'm saying it was the brake. After all, if the chain jammed locking the rear wheel, the rider wouldn't THEN put on the rear brake so I reason that the rider wasn't sensing a locked or dragging rear wheel when he put on the brake.
Therefore the brake was what caused the high side - not the chain. That's my reasoning but I can't prove it even to myself.
Why be born again when you can just grow up?