Scott Westcott suggests that employers ask during the exit interview:

If one person leaving the firm would cause you to think twice about leaving, 
  who would that person be?

Who do you think is next to leave and why?

What could have been changed six months ago that would have kept you from leaving?"

Why didn't you leave before now?

If you were put in charge of the company today, 
  what is the first thing you would change?