Oh, they're just jealous 'cause we won a playoff game.
Seriously though, only bad Cowboys fans I've actually heard are the ones on the radio and other media. The media tends attract bad fans of all teams.
Getting away from the media, the bad fans are usually the band-wagoners for whatever team won the last SB or that they started the season off thinking would. The Saints come to mind, although a lot of their bandwagoners are probably jumping ship right now.