I got into game and

I got into game and i was playing ezreal. I Alt-Tab to message someone on facebook then i go to click on the LOL client tab thing and it wouldnt open it. So i click it again, and again. I was like there is no way that im going to AFK. I relaunch the league program and then it says something along the lines of "Client already open would you like to terminate" I said yes so i sign back in and i reconnect and the second i get into game they remake. and i get an Leave Buster punishment. I have had a problem where i couldnt open the client but i never got a leavebuster from it before and now im pissed.