Hmm, sky.
I don't have that problem. It sounds like something in settings though. It's like your browser is picking up the "movement" (activity) and pushing that back up to the forefront when that's happening. I've had that happen rarely when I've had more than one thing open. Just like when doing something and someone IM's...the IM comes up first and foremost due to the activity.
I've had that rarely happen outside of IM, but never with the site and while in chat.
That would be a bit frustrating if it happened alot.
KD