I'm going to have to side with EaseZE here. It most likely has to do with roaming profiles all sharing the same web/cookie cache.

Or... the proxy is doing it... but I've never had a problem with any of my proxies doing that... at work, home or school...

Like DeadAddict suggested... I would edit your site options and then login manually everywhere. To logout, simply click the logout button. If they are all sharing the same cache, then that should expire that cookie (or however AO does their autologin).