This seems to be a farily new scam. Not sure how many crooks are up to speed on it yet.
There's a video on it at how to stop tabnabbing
It's called tabnabbing (hmmm, bit of a wierd name). Was going to explain it but can't be bothered. Basically if you got tabs on your browser (which most do) then website owners can pull a nasty trick.
Just a heads up for the superbowlers.