I'm not entirely sure, but it seems my webstats provider (which i have been using for the past 13 or so years for several sites) decided to sneakily feed popup/unders to people loading pages where the statistics are counted. They are the only 3rd party service i use on teh chat page, so only they can be responsible for this.
I'll immediately remove the stats code from the chat page. It's crazy really... ive been using them for years, and now they betray us like this.
How long have you been seeing this?Edit
: the stats code has been removed. You should now no longer see the popups/unders. I'll be sure to give that stats site a piece of my mind for sure.