It looks .... Weird. I finally got up the nerve to watch today. Just up to the moment of contact, none of the aftermath. Honestly changed my opinion a little.
I was imagining that Johnson was falling and his neck was at knee height or something.
But nope, Johnson was completely upright in a normal skating posture. Petgrave tries to slide around another player to get to Johnson as Johnson crosses the blue line. Petgrave ends up in front of Johnson and almost has his back to him. Petgrave is off balance and his skate flies up, it looks like a scorpion kick in soccer or almost a superkick in pro wrestling.
My understanding of criminal law is that it should be pretty much impossible to get a conviction. People like to see terms legal terms like "gross negligence" and think they apply to situations when they almost never do.
But that play is weirder than I thought it would be. Between pros and rec league (I play and I live near a rink that is a great place to take my autistic kid to be entertained away from crowds) I probably see 10 hockey games a week. I don't think I've ever seen someone's skate come up like that. So I kind of understand why they're investigating. He either lost an edge and reacted in an incredibly unfortunate way, or maybe he was going for something dirty like a knee to knee and that's why his leg was in an awkward position.
But the whole thing has turned political online now and I'm sure that had something to do with the arrest. Right-wing trolls are flooding hockey forums. My extremely conservative mom just texted me that she is glad he's been arrested.