Brian tried not to invest too much emotionally after Yuna but still felt hurt by Adam's leaving. His ending the relationship is unusual. But his stable is really full these days and having to deal with parents may be too much for him.
Patrick has had two coaches quitting him, once two months before the 2010 Olympics, and once right after Worlds 2012, for similar reasons, the coach being unhappy with his close relationship with the specialist introduced by the coach.
Yes, just like Patrick's mother's interference became too much for Don Laws, and later Christy Krall, it appears Nam's father's interference became too much for Brian

Brian's time is valuable, and he doesn't need to deal with a parent who repeatedly undermines his authority, disrespects him, brings more stress into his life, etc. He is in high demand, and for every Nam there's probably 10 more skaters waiting to get a chance to train with him. So he can certainly afford to cut his losses. Still, I think Brian is easygoing enough that he would have to be pushed pretty far to say "enough."
there are some parents I have NEVER met. I cannot even tell them how their kids are doing...