Demoted for winning a game?

[=) Random Task (=]

I have been promoted after a loss twice. So definitely the last game alone doesn't trigger the promotion or demotion, it is a summary effect of a larger number of recently played games. I assume you can view your hidden rating as a first in first out stack. Maybe a win against a very strong player left the stack and a win against a player with a weaker rating entered the stack. Overall your rating would be lower after the latest win. If this takes you below the demotion threshold this triggers the demotion.