>why is he opening up his own practice?

He is opening his practice only to patients that are comfortable with him and he is comfortable with. That's a different situation than working with who the hospital assigns him. Also he does say they were 'tentatively' willing to still keep him but it sounds like there was friction nonetheless.

I would submit there's a pretty big difference between his long term preference for pseudo-anonymity and the question of what to do, right now, with his patients. He made a decision to leave his practice and transfer the patients to someone else, and start his own where everyone is coming into the relationship eyes wide open. Given the circumstances, he certainly had a choice, yes. And given how he evaluated the costs and benefits for his patients and his career, he felt his only real choice was the latter. Like I said, "forced" is probably a clumsy adjective to use for something requiring a more nuanced explanation.