I listened to a few of the Ringer podcasts on the Boston subject. Simmons, Russillo, Jackie McMullen, Kevin OConnor all have Boston roots and intel. It sounds like Ainge had a bad last couple of years, with Kyrie, not being able to pull off a the home run trade, and knew his time was limited. He also hoped to hand the GM job off to his son, but the last few years prevented that too.
The biggest issue was Ainge would trade anybody at any time and that hurt him with players, especially luring free agents. Ainge also fleeced Philly in the Fultz deal and Cleveland in the Isaiah deal. No one wanted to trade with him.
Finally, Stevens' kids are the age where he didnt want 6 months of travel each year. He was burned out.
They all expected some roster turnover and thought Kemba is untradeable. They assumed Marcus Smart and some prospects would get dealt. Clearing Kemba allows them to re-sign Fournier.
Chauncy Billups and Darvin Ham are getting looks for the coaching position which signals a change towards a former player (player friendly?) type who can connect better. Based on what I have seen, they will hire an outsider who is on another team's bench currently.