It's not league specific, it's online gaming in general that's toxic. For example, I got into the beta for Paladin's (if you want you can buy a founders pack for 20$ and get early access, I find it quite enjoyable), It's by the same people that made smite and it's in closed beta right now. So even though it's in closed beta, it's a FPS game (not a lot of time for typing) toxicity is still rampant throughout the game. It's not every game, but in quite a few.
Personally, I think this has to do with the anonymity of the internet. On the internet, you don't have to think of the other player being a person, they are just a figment, not really alive. That way we justify ourselves in letting ourselves lash out at other players. While it does happen, let's say in American Football, it is actually quite rare that players will lash out at each other. For example, in American Football, if a player makes a mistake, his teammates aren't swearing in the poor guy's face, they will often leave him alone or try to get him to snap out of it and get ready for the next round. The difference is, these guys are physically next to eachother.