The theory -- @Chris Coleman can chime in on this -- is that Nix arrived too late (June, I think?) to really learn the defense, calls, players, etc., and that compromised the defense throughout the season and was partly responsible for Bud moving to the booth during the season: calls weren't getting made in time and getting down to the field in time. Justin Hamilton has been here throughout the last couple of seasons, plus spring practice, so he knows the defense, the players, and the calls, and things will proceed much more smoothly.