Well, you said many times that your therapy rarely involved anything too deep. We could ask, why would someone, who has a full life, social and otherwise, be overly upset about losing a superficial connection? Or even something that used to be quite reinforcing, perhaps especially because it did not require hard work and effort? It is quite likely that your T and you have experienced the whole thing very differently - why he is helpless now that he sees you in turmoil and why you cannot imagine his side of it. He has never seemed to be overly cautious and professional, perhaps he did not like being a T much either - maybe a new opportunity came up and he decided to make a sharp turn.

Another possibility I can imagine is that he actually experienced some serious turmoil and crisis in his own life but did not deal with it appropriately for a good while, and now finally want to really make a change for his own sake. But would not share with you the true path leading up to it or now details of his decision and determination because he knows it's primarily related to his own things.

Not sure this is useful, and I am not a T, but I have had to let go of a few coworkers in my career, usually after many chances, trials and errors to make it work. Some of them I grew to like quite a lot personally, and engaged with them more deeply than average, but the work itself just never worked out. I kinda abandoned a few work projects that way as well, and things sometimes remained unfinished. I think typically when someone makes a decision about such a move, they tend to find it easier to have a poker face and no longer dive into it deeply. It's part of how separation can work, be it separation from people, projects, jobs, leaving an old residence for a new one... It does not mean it was not very important in the past and that it did not affect someone deeply and in lasting ways. It does not mean they don't feel anything at all, but maybe very different feelings from yours. But a professional will usually not share all those personal details with a client.