I could be wrong, but I think the Iron bowl was moved back simply because of the addition of the twelveth game. Originally, there were only going to be 12 games on the years when there were 12 Saturdays between Labor Day and Thanksgiving, everybody plays Labor Day weekend, leaving one week off somewhere along the line. But everybody liked the extra money of the added game, so they decided to go to 12 games all the time. That means on years where there are only 11 Saturdays between Labor Day and Thanksgiving, 'bama and Auburn would have to play 12 in a row to have Thanksgiving week off.