Scout fights with Cecil Jacobs because he insults her father for defending Tom Robinson.
Cecil Jacobs is a town kid, “who lived at the far end of our street next door to the post office.” Scout gets into a fight with him when he insults her father. Scout has promised Atticus that she will not fight, and he said he would “wear her out” (spank her) if she did it. Yet Cecil angered her so much she forgot.
Cecil Jacobs made me forget. He had announced in the schoolyard the day before that Scout Finch's daddy defended niggers. I denied it, but told Jem. (ch 9)
Cecil Jacobs was telling the truth. Scout does not understand why what he is doing is a bad thing.
Atticus explains that all he is doing is defending a Negro, Tom Robinson. He tells her that there is nothing wrong with that. He believes that he must do it, because he could not hold his head up if he did not. He has to do it for his own self-respect, because he has to do what he thinks is right.
In a way, both Scout and Atticus are standing up for what they believe in. Atticus is defending with words, and Scout is defending with fists. Atticus is trying to show her that his way is better.