Ballmer promoted Muglia again a year ago, making him one of just four divisional presidents and adding the Azure Services Platform to his portfolio. Since then the presidents of the entertainment and business software divisions have also left, in addition to Ozzie.
Some speculators tag Muglia with the failure of Azure to overcome Amazon Web Services as the leading external cloud service, or with not pushing the server division quickly enough toward the cloud.
Others note Muglia's consistent support of cloud in general and Azure in particular, but don't pint to a specific failing that may have prompted Ballmer to act.
Given the brain drain Microsoft has been suffering during the past few years, and it's lackluster showing at CES and in competition in nearly every area of its business, there's a limit to how much blame can be pinned on Muglia.
Microsoft has lost a lot of ground to companies like Amazon, Rackspace and others moving aggressively into the market for cloud-based services, but not usually for technical reasons.
More of the holdup has come from bits of archaism like keeping licenses yoked to a particular physical machine, failing to strike a balance between offering apps as services online and letting VARs sell them directly, "innovating" instead of innovating, following the competition (and doing it badly,) and failing to make a convincing case that Windows can or should survive as a desktop/tablet/smartphone operating system for the forseeable future.
Muglia may have taken part in discussions on many of those questions and may have slowed some of them down, from Ballmer's perspective.
The problem isn't within Muglia's portfolio, though.
It's somewhere higher up, somewhere memos come from. Someplace monkeyboys used to come out to play, but somehow don't anymore.
I'm sure Ballmer will be able to figure out where the problem lies, eventually. Until then, look for a lot more senior Microsoft executives moving on to "other opportunities," possibly with Microsoft competitors.