This happened to me yesterday when I was on GF at work. I had been on about 5 minutes - went to reply to a thread and it asked me to sign in again!
I was using Explorer at the time.. Not sure if that made a difference?
I always have the "remember me" box ticked and I never have to log on unless I log out on purpose (which I do often). However in the past days I just get logged out without a warning. The good thing is that when I'm typing a reply, it doesn't get lost after I log back in.
ps. And it just happened again as I was posting this reply, haha.