The person who really does the most to persuade the mob to back off is Scout, even though she does not know what she is doing.
What Scout does is to talk to Mr. Cunningham, thus singling him out as an individual and making him think as an individual.
Psychologists tell us that people behave one way when they are acting as indiviudals and another way when they feel like they are part of a faceless mob. Cunningham was part of a faceless mob until Scout talked to him. Once she did, he went back to being an individual. He then realized what he was doing and backed off, taking the mob with him.
Scout talks to Mr. Cunningham and makes him think for himself, not just going with the crowd. He was not aware of what he was about to do until Scout made him realize that he is an individual and going with the crowd at the moment would be the wrong thing to do which is why he backed off.