"Desync!" Message when reconnecting to game

While waiting for a game to load (Team League); my game instantly closed with no error message. When I started up the game, I was given a message that just said “Desync!” and only offered me options to quit the game. I forcefully powered off my computer, started it back up, and it let me successfully reconnect.

Why did this happen? It ruined my ranked game and my team mates were not happy.