I study and write about the social system we have in the US, which is based in a patriarchal dominance hierarchy. Since Black women are ostensibly "supposed" to be the very bottom of the pyramid of power and privilege it apparently feels like a great injustice to those who can't fathom how someone "below" them would dare to criticize them - even though it's justified. And yes, it's extremely fragile and adheres to this pecking order view of the world where one is expected to take abuse from "above" but then also allowed to dole it out "below." It just really upends some white women's worlds to have this hierarchy challenged.
And understanding it doesn't make it any less objectionable. Sorry you have to deal with that shit.