If one of google's working theories is they should hire people to keep them away from other organizations then it makes sense they don't fire until they see their competitors start.
Is this really a surprise to anyone at this point?
They weren't axed. They are choosing to leave. They probably reached the point where all the incentives to stay were earned and have no other reason to stay. The Fitbit acquisition was completed 3 years ago so it is quite likely they have reached full vestment.