See title.

It just happened. It has happened before.

I am pretty shure that is a feature not a bug. It has something to do with account security I think.

I think they do it for security reasons, usually around every month or so. I actually manaually log out and back in again every 4 weeks to avoid this, because I don't want it to happen while I'm in the middle of a quest or something. It seems to have worked so far.