Turnover
I believe that a big reason for so much movement at the regionals is due to people jumping into the industry only to realize that it isn't as much fun as the pictures in flying magazine would suggest and they quickly return to their old life.
Easy come. Easy go. With so little time invested into the profession these days it must be easier to leave once the reality sets in. 15 years ago it took 5 or more years of professional flying just to reach the interview room of a regional. Now all it takes is 90 days and cold hard cash.
SkyHigh