I can say from personal experience that Apple started losing people the moment they released the App Store but refused to allow employees to participate. They created an incredibly exciting platform for development and then locked their best people out of it. No wonder some of them left.
The problem is now magnified because today there is much more uncertainty in regards to Apple's future. Jobs is gone, Forstall is being moved out with no obvious successor, Google/Android is taking the lead, the stock price is stagnating, iOS is losing its edge, PC sales (including Apple's) are slowing ... it looks like it could be the beginning of another "beleaguered" era soon. Who would want to stick around for that?