Mech springs get tired, plus the jockey wheels look pretty worn to me. Both those things could easily lead to the chain jumping and getting stuck. Chain getting jammed is definitely the cause of the crash. Glad it wasn't worse, it's a horrible feeling. I had a similar one a couple of years ago when the chain on the road bike snapped when I was out of the saddle. Straight down before I knew what had happen and right onto my two ride old helmet.