He fired Franklin to cut his losses. It's obvious Franklin was a complete disaster as OC at Auburn and Tubs is trying to salvage the season. This also deflects some of the criticism of the offense for the rest of the season. Even if it still sucks, it'll probably show at least a little improvement, and Tubs has already done about all he could possibly do mid-season, so the critics will not be nearly as loud as they'd be if Franklin were still on board.