The Shearer negotiations would seem to me to be the most obvious reason for the sale.
I'd hazard a guess that Shearer has all but said no to taking over full time with whatever restrictions Ashley has placed upon him should he take over (no money for new players, have to sell most of the 1st team, etc.). Ashley's simply getting out before everyone starts demanding he f*** off for screwing up another managerial appointment.
First Keegan, possibly now Shearer. All he'd have to do next is hit Sir Bobby whilst driving out of the ground for the trifecta.