He's a Microsoft-lifer who knows the business cold. He's shown no signs of being burnt out, or sick of the job.
One source familiar with
In the press release, Ballmer said, "There is never a perfect time for this type of transition, but now is the right time ... We have embarked on a new strategy with a new organization and we have an amazing Senior Leadership Team. My original thoughts on timing would have had my retirement happen in the middle of our company’s transformation to a devices and services company. We need a CEO who will be here longer term for this new direction."
We expect to hear more about his depature, but this really sounds like the most logical reason for him to leave now.
The last year has been very busy for Ballmer.
He's been transforming Microsoft from a software company to a "devices and services" business. This means services like Office, Cloud, Windows, supporting devices like the Surface tablet, and PCs from partners like HP, Acer, and Dell.
In addition, he reorganized the company to fit that vision. He restructured the company to be more collaborative. He has an operating systems group, an applications group, a cloud and enterprise group, a devices group, a marketing group, and so on. Those groups are all supposed to work together to create "One Microsoft" as Ballmer put it.
At the same time, Microsoft released Windows 8, and the Surface tablet.
With all that work done, a natural transition point emerged for Ballmer. He had enough runway to spend the next twelve months working with the board to find a replacement.