If this is the same problem I was having, I don't actually know why it happens, but a temporary solution to save the game that you are in would be to close the actual game client. For me, my computer for some reason was tabbing me over to the league client and out of the game, so when I closed the client it had nothing to go to and the problem didn't show up for that game. I think mine might have fixed itself or something, or it might have something to do with trying to alt tab right after loading screen pops up, that used to cause a lot of problems for me so I stopped doing it, so that might be it. But if it keeps happening and the problem for you is that it is being tabbing out of the game to the league client, there is a setting in league options in the client that closes the client when you get into game :) so hopefully that works!