I'm guessing...
I thought that somewhere in your profile there is an Always Log On button, or something similar, but I can't find it now. Mind you, that should only log you on automatically, not leave you logged on when you aren't here at all.
Not convincing. Another guess then...
Maybe you'd been logged on earlier, and left the site without logging off. But the system doesn't know you've gone - as far as it knows it's just waiting for keystrokes from you, but you've actually shut your machine down. I'd imagine there is a timer to expire a user session after a certain time period. And maybe that cut off time hadn't been reached...
