I've suffered similar problems when I've been using someone else's computer, since I don't want my Org details to be remembered anywhere that isn't my own computer.

When the "Remember Me" box isn't ticked, the .Org automatically signs you out after prolonged inactivity, that is to say a period of time where your computer hasn't loaded a page at the forum, whether that is a thread, private message, user profile, previewed post or anything else. :3