I would say on Microsoft's part it was when Ballmer was ousted and Nadella took the reins. Nadella seems to be genuinely open to working with instead of against other companies and open source projects. I think he realized the ship was sinking, and he's doing what he can to save it, throwing out old and dead business practices in the process.
I actually like the "new" Microsoft and the direction it's heading, and I'm still trying to wrap my head around that fact.
Ballmer was at the helm when this train started rolling. It has taken years to get here. When Hanselman said to Miguel, "We did it." on stage at dev connect, I took that to mean something much larger than open sourcing Core.net. There has been a push from within on open-sourcing and pivoting on their approach to many of their lines of business that has taken so many people so many small battles, that I would sya Satya's ascendancy was the coming out party for a movement vs. some catalytic event.
You're right of course, these changes didn't happen with the flip of a switch. It just seems that so much good has happened at/from Microsoft in the relatively short time since Nadella took over, it's almost as if that was the moment the shackles were taken off. Obviously the ground work was laid when Ballmer was still in charge, but it was almost as if he resisted the changes that were coming down the pipe to the very end.
I actually like the "new" Microsoft and the direction it's heading, and I'm still trying to wrap my head around that fact.