a Bug? : I was enrolled to two leagues simultaneously

[TheGreatAnt]

And besides, perhaps the person got bumped from that league, and so whichever game was finished next for you, you would get promoted because of that other person's loss, you automatically are in the next league, you just had to finish a game for it to tell you that you got promoted. And it just happens that you lost that game you got promoted on.

It's not that hard to believe. Just realize there are more players than you getting promoted/demoted. There are a lot more players than there are on the forums.