People internally has been pushing Microsoft the open source way (some evangelists of MS and .Net). They have gotten greater influence into the firm, because of success and a allround positive influence (and Microsoft didn't had any appeal to a great group of developers because of closed source and patents).
The similarity between the new CEO and those people where greater then with Balmer. So it's a logical step into it's evolution.
It doesn't mean that Balmer hold them back at all, they could thrive under Balmer, but perhaps he hasn't had any experience with this business direction or couldn't grasp the concept of it. And as a result, someone else was more fit for the job.
Steve Jobs promised that FaceTime would be an open protocol... never happened.
AirDrop is yet another closed protocol.
XPC is a new protocol for sandboxed processes to communicate with each other, it's basically distributed objects. Again, a secret even though things like CORBA, Java RMI, etc. have been around for years.
But SQL Server Management Console supports obdc, LocalDB (like SQLLite) and SqlServer (MySQL) at once.
But it's on the Microsoft platform though.. I wouldn't know another reason.
It would mean binning the project and starting from scratch privately right? At the risk of being accused of stealing the open contributed code.
However, code that's been distributed under an open license stays that way. So Microsoft could add a commercial license, and even stop making updates under an open license. But if they do that, then there's nothing stopping the community from moving forward with their own fork of the open source version.
If I recall correctly, the Darwin operating system might be a good example of how that sort of dynamic could play out.
(I'm very sure however that they have this worked out. )