Among the possible reasons offered by Wonkette:
"Trent finally had enough of Mitch McConnell trying to play footsie under the table at leadership meetings."
"The other Senators kept making fun of his and his wife's matchy names: 'Trish-Trent! Trish-Trent!'"
"He didn't want to be the last racist left in the Senate, and Byrd was looking damn old ..."
Read 'em all at Wonkette.com.
By the way, Lott says his resignation has nothing to do with those pesky anti-lobbying restrictions on former congressmen that take effect next year. Just so you know.