why did calpurnia hit scout?

Calpurnia smacks Scout to emphasize an important point.
In the story, Calpurnia is a mother figure to both Scout and Jem. The text tells us that Calpurnia has been with the Finch family since Jem was born. Although Scout does not remember much of her mother, Jem does. We learn that Scout does not remember her mother, because she was only two years old when the latter died of a heart attack.
As one of the authority figures in the children's lives, Calpurnia works hard to instill good values in Scout and Jem. From what we can see of the interactions between Calpurnia and the children, the housekeeper takes her responsibilities seriously. However, Calpurnia also has a soft spot for her young charges. When Jem turns twelve and becomes difficult to live with, Calpurnia becomes an important source of comfort for Scout. She welcomes Scout into the kitchen and spends time with her. Calpurnia is an important presence in Scout's life.
When Calpurnia smacks Scout for her unkind remarks to Walter, she does it to emphasize an important point. Essentially, Calpurnia wants Scout to be kind to her guests and, most importantly, to treat people who appear to be her social inferiors with compassion and understanding.

“He ain’t company, Cal, he’s just a Cunningham-” “Hush your mouth! Don’t matter who they are, anybody sets foot in this house’s yo‘ comp’ny, and don’t you let me catch you remarkin’ on their ways like you was so high and mighty! Yo‘ folks might be better’n the Cunninghams but it don’t count for nothin’ the way you’re disgracin‘ ’em—if you can’t act fit to eat at the table you can just set here and eat in the kitchen!”

After this soliloquy, Calpurnia smacks Scout to emphasize her point. As for Scout, she spends the remainder of her meal in the kitchen.

Calpurnia smacks Scout as a punishment for making rude comments about Walter's eating habits at the dinner table.
In Chapter 3, Jem invites Walter Cunningham Jr. over to eat dinner with his family, and Walter proceeds to pour syrup all over his meal. Scout is disgusted at Walter's eating habits and rudely comments on the fact that he is pouring syrup all over his meat and vegetables. Scout embarrasses Walter by asking "what the sam hill" he is doing. Calpurnia then requests Scout's presence in the kitchen and proceeds to reprimand Scout for her rude remarks. Calpurnia chastises Scout for not treating Walter with respect and tells her to stop acting so "high and mighty!". Calpurnia also makes Scout finish her meal in the kitchen and smacks her while she is walking into the dining room to retrieve her plate. While Scout is eating alone in the kitchen, she contemplates on how she will get revenge on Calpurnia for punishing her.