Mrs. Dubose is different from the other insults because she always has insulted the Finch children, and just uses Atticus’s defense of Tom Robinson as another barb to throw at them while she is under the influence of morphine withdrawal.
Mrs. Dubose has always been mean. In fact, Scout describes her in the beginning of the book as “the meanest old woman who ever lived” (ch 1). She is also very old, “close on to a hundred” (ch 10). Since the children cannot go to town without passing her house, they have to put up with her. Atticus warns Jem to treat her well.
"She's an old lady and she's ill. You just hold your head high and be a gentleman. Whatever she says to you, it's your job not to let her make you mad." (ch 10)
Mrs. Dubose always shouts insults at the children as they go by. It was only a matter of time before some of her insults centered on what Atticus was doing, because she would grasp at anything to insult them with.
"Yes indeed, what has this world come to when a Finch goes against his raising? I'll tell you! …. Your father's no better than the niggers and trash he works for!" (ch 10)
Mrs. Dubose is actually very ill. Atticus explains that she is under the influence of morphine, and does not know what she says. She is trying to wean herself off of it, and it makes her an even angrier person.