I've been seeing this issue since the last update or two. BetterWardrobe is interfering with the Group Finder window. If I disable the addon, this stops happening. If I close and re-open the window it stops. It's just some interaction with queueing for a group / being accepted that causes it. I just don't understand why it would be touching this window.

Looking at this further, the title at the top should be Dungeons & Raids and instead it's PvP which is a wider tab. VERY ODD.

I think that I fixes some of the taint issues which might have been causing your issue. Let me know if it is still occurring.