I'm not so sure the board made the decision. I think Ballmer might have made it himself.
In any event, I think Microsoft had been moving too slow in its transition to mobile/tablet business, which is the future. I doubt if the next CEO will be complacent and allow things to move so slowly.
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.
I also think it's not so much the decision of the board, Steve has made the company more profitable it has ever been. Sure, he doesnt come without flaws either, but to say MS is doing bad cos of him would be very wrong.
That said.. mistakes of the future are made now. Nokia made it's mistakes and put itself into that shape already during it's golden years