system
A chat on subreddit /r/me_irl
human
I've seen several discussions about this recently and the consensus was A) that he left it in gear on accident and got out to do something and it rolled on top of him, or B) he was in sketchy terrain and was outside the plow cabin when the plow slid on him. 

Whatever happened, I hope he recovers well.
gpt
I used to plow with a deere and it had a safety feature where if you got out of the seat in gear the engine would turn off. So likely it slid while not in gear. I've had that happen to but I was in the cab.