Why, specifically, does the mob break up?

Looking for a friendly face in the group, Scout recognizes Mr. Cunningham, the father of Walter from her class at school. Trying to be cordial, she innocently begins to talk to Mr. Cunningham about how Walter is a good boy, and recounts how they invited him home for dinner one day, and asks Mr. Cunningham to say hello to his son for her. Then she tries to engage him on the topic of his entailment, which she heard her father mention once, but notices that everyone is staring at her. Mr. Cunningham bends down to Scout's height and says, "I'll tell him you say hey, little lady." The men decide to disperse, and go home in their cars.