maybe you havent change your clan since you started this game. the clan who has the most cards in a fight will have the background. for example,
the background of the Uppers is the building with the Uppers sign on it.
the background of allstars is the stadium.
the Freaks is the circus with the big clown face.
the sahkrom is the building like tower with many sahkrom signs on it.
so observe it...