You want to know why people are leaving? Because game development sucks, plain and simple. There's long hours, ridiculous deadlines and you don't get paid much. Developers are a hard group to please in general. It's hard to keep us interested for more than a few weeks, let alone a few years. Combine that with the fact that game development is incredibly difficult (especially with ND's reputation), it's no big mystery that people are leaving.
In other words, they're leaving for the same reason anybody would leave any job: they want a better job with fewer hours and more pay. :lol: