He wants a promotion. And when you consider the likelihood the Cowboys could have 3 new starters in the secondary next year...he probably does not feel like "starting" over. I think it is fairly normal not to want to be a position coach for an extended period.
Makes sense that he'd take off. If Claiborne and Carr is gone and we have two rookies, he's really going to get the blame when our defense gets lit up. Best get out and go for an upgrade while the opportunity is there.