When Steven Sinofsky's departure from Microsoft came out, it was framed by Microsoft in a way that implied he might have just left, or that that was at least a possibility. Now, ExtremeTech is reporting that was not the case; Sinofsky was straight-up fired by Ballmer.
While the report indicates that former heir apparent Sinofsky was quite suddenly and explicitly fired as opposed to choosing to leave, the precise reasoning behind the decision isn't known. Sinofsky was known for being difficult to work with, and reportedly refused to put tablet support into Windows 7 and dragged his feet on tablet projects in general. That being the case, it's easy to see how he would not fit well into a Microsoft that's making an effort to develop a more consolidated product and promote cooperation between arms.