Jack spanks Scout because she beats up Aunt Alexandra’s grandson for calling Atticus a cruel name. As a result, Uncle Jack spanks Scout without hearing her reason for acting so violently. Later when Uncle Jack finds out the whole story, that Francis had been calling Atticus cruel names for defending Tom Robinson, he regrets his actions.
Atticus knew that defending Tom Robinson would cause trouble for his family and that people would talk poorly about him for his decision. He has warned Scout not to fight with people if they talk badly about him.
Uncle Jack spanked Scout after she had "split my knuckle to the bone on his (cousin Francis') front teeth." Scout never got along with her cousin Francis, and when he called Atticus a "nigger-lover," and then repeated the remark to her, Scout decided to take action. She waited for Franciks to separate himself from Aunt Alexandra, and then she let him have it. Jack pulled her off him, but when she tried to run, her uncle spanked her, leaving "seven or eight red marks."
Atticus wasn't too upset. He chuckled. "She earned it, so don't feel too remorseful," he told his brother.
Uncle Jack spanks Scout for using very abusive language and nearly breaking her cousin's teeth. He spanks her more for her abusive language since he felt he had already made it clear to Scout not to speak in such a way after Christmas dinner (when she had requested for him to pass the 'da*n ham'). Scout is scared when put on the scene before everyone and knows she's in for it, but her attempt to escape proved to be her undoing. She examines the damage to be only '7 or 8' red marks.
Atticus seems to think he too had warned Scout many times not to be hot-headed. He feels that Scout earned her spanking, no matter what the reason was, and thinks that Uncle Jack is just commonly regretting his deed without reason. But he knows that this is only the beginning of what will be an ugly phase of insulting for his children and hopes Scout has learnt her lesson for good on fighting with her fists and not her head.