You might remember the context of it. After 1971, Auburn lost some of its best players its ever had after getting bludgeoned by Alabama 31-7. Everyone predicted Auburn to be terrible in 1972. New players and Alabama was supposed to be amazing.
Once the Iron Bowl arrived, Auburn was 8-1 and Alabama was ranked #2 in the country. Even though Auburn had a decent record, Alabama was still heavily favored. The hype for a big Alabama win was given a steroid shot when Bear Bryant said "I'd rather beat that cow college once than beat Texas 10 times."
What happened during the game was nothing short of miraculous. Auburn blocked two punts in the 4th quarter and returned them both for touchdowns. The game is now infamously known as the Punt Bama Punt game.
Afterwards Bear Bryant said he wouldn't refer to Auburn as a cow college anymore.
The t-shirt is a reference to the 1972 season.
At least that was Tiger Rags' explanation.