I don't know that there was an official reason. My recollection is that it was simply announced. He's not retiring. Even if there was a difference of opinion (and we don't know that either) he clearly doesn't want to do anything to damage the work they're doing. All we know is that he is no longer officially associated with them.
He said he wanted to be able to go sailing and have a normal life. It sounds like he is still seeing patients with a long waiting list. Starting a whole institute going requires a huge investment of energy for whoever takes it on.