[_ah]

If everyone has good things to say on their way out the door, that means that Manager A is a "Good Person". So I can think of two reasons to leave:

1. Manager A is a Good Person but actually an incompetent manager. Everyone likes him, but he's terrible at teaching people and developing their careers. Eventually, people go elsewhere for growth.

2. Manager A is actually fantastic as career development, so much so that he convinces his team that they are capable of so much more. He builds them up and then sends them out into the world with his blessing to do something bigger.

FWIW, I think option #1 is most likely.

For Manager B, this is a person who is steady and low drama, but also not super invested on the personal level. Likely does what he has to for his team, and the folks who stay are not super ambitious but also appreciate the low drama environment.