I don’t have much insight into internal Google politics but there seems to be a rash of articles and blog posts over the years about prominent folks seemingly and abruptly announcing their exit. What is behind this trend?
Corporation rot. If you are successful when you are relatively small and hire the best and brightest, then you scale out in numbers, you go down as this model is not scalable. When you have tens of thousands of people, you bring "professional management" that are bozos (this is a quote from an interview with Steve Jobs), the organization first starts to decline as an average (cannot find tens of thousands of really bright people, there are not enough in the world) and second it starts to rot as the bozos have a huge negative impact. When the bozos change the culture for the worse, top people leave, average employee is declining even more. It is a race to the bottom, not in pricing but in employee quality.