When I leave my MacBook Air overnight to say do a Time Capsule, I see that it will log itself out unexpectedly and automatically. Kind of wierd, my MacBook Pro doesn't do that. Some users are having the same issue with the "Mac Mini":http://discussions.apple.com/thread.jspa?messageID=7766844 and the only real diagnosis is:
One thing I think is worth doing is looking in the logs on the system for entries after this has happened. Logs entries are often hard to interpret and can be very misleading, but opening Console (in Applications>Utilities) after a software crash of this sort will often yield at least some clues in entries from the time of the event.
That said, I suspect there may be a relatively simple cause of the problem in the case you describe. There could be nothing much more than a permissions or prebinding issue on the system. I would use one of the simple 'cache cleaning' type utilities such as YASU (http://jimmitchell.org/projects/yasu/) set it to all tasks and then close everything else down until it's finished. That would deal with any incorrect permissions, out of data prebinding, and a host of other minor ailments that can cause apparently quite serious system instabilities. After that, favorite would be a broken property list file or a damaged or conflicted font.