While it might seem that a lot has happened since Satya Nadella took over as Microsoft's new CEO earlier this year, including the open-sourcing of the .NET frameworks, it's really the result of a process several years in the making. Mary J. Foley elaborates on ZDNET:
Somasegar said management was aware that Microsoft hadn't embraced open source in needed ways. The thinking was that Microsoft should start out by open-sourcing higher levels of the .Net Framework at first, and then consider gradually move down the stack. But there were debates about whether Microsoft should actually take contributions from the community, as is expected under most, if not all, open source licenses -- or simply make the code available for viewing but not modification.
If you're into .NET or open source software it's worth a read.