This incident occurs in Chapter 16 of this great autobiographical novel and concerns the author's first experience of working as hired help in a white household. Although her job starts off alright, when her employer starts calling her "Mary" instead of her proper name, Margaret, the author recognises that this represents an insult that cannot go unchallenged. Renaming your black helpers is presented as being a perfectly normal practice in that time and place, as Miss Glory tries to console the author by saying that she used to be called Hallelujah before her employer called her Glory, but even so, Margaret recognises that this name change is a symbol of the absolute power that whites have over blacks, and she refuses to let her identity be shaped in such a way. Note how she responds:
I had to quit the job, but the problem was going to be how to do it. Momma wouldn't allow me to quit for just any reason.
The author therefore recognises that her only response to being given another name is to leave her employment, and she therefore "accidentally" breaks a favourite casserole dish of Mrs. Cullinan's so that she can be fired and leave with a perfect excuse.