I believe I suffered from "MMR de-- what did they call it... "MMR decay" or some shit.
"MMR" is the secret number that determines what league you're in, and how good it thinks you really are. When I stopped playing, my "MMR" was solidly in Gold. However, then I stopped playing because drinking vodka was much easier and I was in Masters.
So I guess in between then and now, my MMR has "decayed" to the point where they figure I'm back in Bronze.