That is nasty. Awesome that you were able to go back and do the forensics work to come out with a cause and analysis. I'm glad it wasn't any worse for you. That shoulder edge could have farked you up a bit more too. Yuck.
My big concussion left my training partners behind combing the road for any shred of why I would have gone down, and the only explanation turned out to be wildlife. No evidence of any other causes.