What happens between Scout and cousin Francis? Why?

After Christmas dinner, Francis and Scout are outside in the backyard. Francis says that Atticus is a "nigger-lover," and that now Atticus will be the ruination of the family, who won't even be able to walk the streets of Maycomb. Scout patiently awaits her chance, and then punches him squarely in the mouth. Francis screams and everyone comes outside. Francis says Scout called him a "whore-lady" and jumped on him, which Scout does not deny.