I've always heard the reason is that Auburn is the furthest east of all the SEC West Schools. No other logic than that.
But if it does happen, Auburn gets fucked in the deal. Playing UT, UGA, UF, and Bama every year gives them the hardest road to the SECCG of anyone (unless UF keeps it's yearly game with LSU, then they get fucked like we do).
How about a complete shake up:
SEC North: UK, UT, UGA, USCe, Ark, Mizzou, Vandy
SEC South: UF, LSU, Bammer, AU, aTm, MSU, Ole Miss