There's a simple explanation for this phenomenon which isn't very exciting, I'm afraid.
When there's such an unusually large number of "guests" logged on a forum all at once, it's very likely that most of those hits are being created by search engine bots trawling the web - Google, Yahoo, and so on.
Sometimes it even shows the bots are not only viewing pages but doing certain actions like sending a PM when we all know they're not able to do this if they are unregistered. Quite bizarre, really.