I don't think the board made the decision either.
I think they may have pointed out certain failures he had and asked him if he could lead us into the future. If so, the next "wave" of changes that mightreally gain steam in a year or so would take several years to complete Was he willing to commit to the entire term?
If not, because the guy really does love Microsoft, he may have decides that it wasn't fair to MS foe him to leave in the middle of the next wave. But he probably also promises his family that he'd retire somewhat soon.