The only reason he would be let go so suddenly before the holidays are three reasons.
Personal misconduct
Extremely unprofitable and need to make shareholders happy before holidays.
Already planned and they wanted to kick start the new year with the changes already in place.
I'm doubtful of the third option only because Rory is abducating his positions across the board - stepping down from the position of President and CEO as well as from his Board of Directors position.
Had this been an intentional transition of CEO responsibilities then Rory would have likely been retained in his Board of Directors position for a while longer.
The only thing that really makes much sense to me here is that Rory must have just learned of having some sort of terribly critical medical calamity (cancer or some such) that requires him to basically immediately stop working and seek aggressive treatments that for the most part will render him incapable of performing his job effectively.
If that were the case, I could see why AMD and Rory might not want to communicate that to the public as it would certainly rattle shareholders even more in the sense that it would be tantamount to saying "we are throwing Lisa Su into the CEO prematurely because of Rory's medical situation, if only she had more time and experience before we promoted her on our original schedule, then the company would be better off".
By keeping it under wraps, it would help reduce the perception that would otherwise exist in terms of just how ready Lisa Su is or was for the sudden promotion to the top job.
That is speculation, of course, but if it is true then I hope for the best for Rory while he deals with whatever manner of medical emergency has beset him.