"I also wonder why an AC wouldn't want to tell their mother what they had done wrong. If there is already Estrangement, there's nothing to lose, just closure for both parties and the AC can tell them direct they don't want to see their parent again. At least this would put an end to things. "
I agree, Joyfulnana - just break the NC "rule" long enough to let the parents know the reason for it. Or better yet, do that before going NC.
However, in some cases, it will just lead to arguing, denial, etc. And the parents will still come away saying, "don't know why." Also, sometimes, they have been telling the parents what the issues are for months and the parents ignored it, that's how they ended up with NC (not saying that's true of anyone here).
Still, I think it's only fair to make sure you (general AC)let them know what happened and maybe even give them a chance to "fix" it if possible. In fact, think you'd want them to know why. But maybe that's just me.