OOh! User You're going to have to have a long think and then probably a long and slightly uncomfortable chat with your DH.
My SIL screamed something similar at me, 20 years ago. BIL (DHs brother) added that I would never be family as I had only married in and so I should just fuck off when family discussions were being held - SIL was obviously not included in this exclusion.
I was the one who tried to persuade DH that he didn't have to cut them off because of it, I would just not rise to it, nor go round again. But he was adamant and we have had very little contact, especially since the death of their mother.
Your DH has probably held his tongue, bitten his lip on many occasions when you have allowed your DM more control over you, and therefore him and your DCs, than is right / comfortable / acceptable.
You can't just say "Oh it was an argument and that always takes 2" and step aside. This is your mother and your husband! You have to know, deep down, if anything he did deserved that response. Yu also have to know if your DM is too enmeshed in your life. Your posts say she is - you keep in touch to avoid fall out?!?
This time, rather than ignore it, use your husbands request to properly explore what YOU would really like to happen with your relationship with your DM.
Is your DH right, would you be happier if you could gain more distance, less of a parent : child relationship with her?