kkb, while I'm no expert on this, I believe that the board auto-logs you out, if it sees no activity after 30 minutes. So reading a long thread and/or taking a long time to reply, means that when you hit the "back" button/key, you are auto-logged back in and your cookies are refreshed.
Until Firefox 1.0 came out, I always used to "lose" my unread threads.