@Chadk: Muglia's email is sort of ambiguous about it, but Ballmer's earlier email clearly states that it was his (Ballmer's) decision to change the leadership at STB and that, in response to that, Muglia decided to leave Microsoft.

Of course, that still leaves the question what motivated that decision. I'm sure plenty of people are already frothing at the mouth to explain this as Muglia being outraged by Ballmer's idiotic decision to replace him and quitting in a huff, but it might as well be that Muglia agreed with the change and then decided to take the opportunity to head elsewhere.