I had noticed this before (being logged out, when I hadn't done it myself) but I cant recall the circumstances
I think it was IE7 on winXP but cant be sure, and I think it was Google Calendar (as opposed to docs)
You say 'some' users have issues? Problems that are not consistent (not repeatable) are the worst kind.
How about trying a different browser?
Google Docs logs them out when they are going from one tab to another.