According to this article, which states “Redmond Refugees Driving Open Source”, many Microsoft alumni are getting into the open source field.
I can think of many reasons, but one thing that sticks in my mind is the copyright assignments that companies require. That and the Non Disclosure Agreements can make anyone want to work for an open source paridigm. The agreements that people have to sign for closed source projects are, on the surface, quite intimidating, and if you can avoid signing them, you are much better off.
Open Source eliminates the whole point of signing your life away, because the source is, well freely available.
Of course some of the most pro Microsoft people that I know are people who never worked there, and some of the people I know who have worked at Microsoft are a little more open to other choices.