why do they play that? Significance?
"Happy Days in Dixie" was a popular song in the late nineteenth century before it was waxed and released as a 78 rpm disc in the following century. During the 1890's, some fraternity brothers at UC Berkeley rewrote the words, turning it into a football song, and it was published in the school newspaper. The tune was eventually titled "Palms of Victory" (no relation to the gospel tune of the same name). The band started playing it at football games, but superstition began to follow the song due to several losses following its performance by the band at games.
It was decided to only play the tune after victories. However, superstition continues to follow it--playing "Happy Days in Dixie/Palms of Victory" before a game is believed to bring bad luck, as in 1992, when the band played it at an alumni event just before the team got trounced by Stanford 41-21.