Do you remember the articles about people leaving Naughty Dog in the last months and people..being worried about the state of Naughty Dog?
Some people mentioned it in comments already...developers come and go and come and go..there is a big fluctuation in the video game business and if you are familiar with the mobygames database and looked up many pages of developers, concept artists, character designers etc. there you might seen that almost no guy who worked in the video game industry for 10+ years stayed at one place.
People, especially outside Japan were it might be more traditional, change from 5 years at Activision to 3 years at Electronic Arts and now they are working for Sega, Namco or Ubisoft.
Its nothing really extraordinay and I am sure we will see a ton more developers changing the fronts.
Nate Wells, former Art Director from Naughty Dog who left the company in April 2014 to join Giant Sparrow games is back at ND and working there now as Lead Artist.