Very often it’s the final straw.
My brother tells everyone who’ll listen I went NC with him, and two of our siblings, because he didn’t like the colour of my dress and said so when asked.
Now in a way he’s right. I did. His friend sniggering and saying “do you like lyra’s dress?” leading to his reply about how hideous the colour was, how much it didn’t suit me was the thing that made me say “I’m not listening to you anymore”.
However, it completely ignores, conveniently, the years of constant belittling, put downs, snide comments and threats of violence.
Very few people go NC go trivial reasons. It’s a huge thing and it’s not easy.
Just one thing though OP - you say your grandmother was horrid to your mother, but nice to you so if your mother had gone NC she’d have deprived you of a grandmother... that’s not right. It would have been your grandmother, through her actions toward your mother, that deprived you of that.
You might also want to think about how difficult it must have been for your mother to see her mother make effort with you that she wasn’t, to your GM, worthy of herself. That’s a very difficult position, and imo is sometimes a way of subtly continuing the abuse. “look at how nice I can be to people who are not you” is one of the hardest things about abusive parents imo.