In an effort to close an exploit discovered in the Combine's Login System that would allow someone to potentially hijack your session (This has not happened so far, but we prefer to be pre-emptive in these things), we have implemented a change to ensure that the person behind the session is who they say they are.
As a result of the upgrade, you were all required to login in again.
Edit: Forgot to add in the upgrade to the login cookie rubbish, so if you found you were being logged out every few hours, that was why! Should be fixed next time you log in. Sorry for the inconvenience.