system
A chat on subreddit /r/NoStupidQuestions
human
From what I've seen it's a complete freak accident. The way he hit at the exact time he hit as his heart was pumping contributed to him going down. 

Completely healthy 24 year old on a clean hit just going down and immediately requiring CPR. Horrifying shit to witness live, I can't imagine what the players went through
gpt
Ya it happened to Chris Pronger in hockey. Hit in the chest with a slap shot.