Rivers points out that he did leave for what amounts to a promotion, since not only is he head coach but also VP for player personnel. He obviously wasn't going to get power over personnel decisions while Ainge was running the show in Boston. I'd even say that Doc and Danny felt they were going to butt heads and have some ferocious arguments over players if they were both around for a rebuilding period. So, in some ways, it's like saying an assistant going for a head coaching job elsewhere is walking out on the team.