Mobas (and team games in general) have an inherent issue where people dont like to be losing, and when they are, they usually find an outlet by blaming the rest of the team.

Its a pretty huge problem in league because

1. Behavior problem. Riot has like never addressed the toxicity issue. They've basically said heres some banned words and thats it. People can get by either griefing in game (which has been essentially unpunished) or by typing in non bannable ways (i remember seeing someone using europe as a uhh... You know)
2. Game design problem. League is pretty punishing when it comes to losing. You're stuck in a game you dont want to play, esp if your team holds you hostage by not surrendering intentionally. There's no fast way to get into a new game. You have to finish this, lose lp (ranked point system in league), and then possibly run into the same situation again. This isn't really riot's fault. This is just a flaw of mobas that no ones learned to really fix yet