What does Atticus think the children did that changed Mr. Cunningham's mind in To Kill a Mockingbird?

Atticus says that Scout reminded Mr. Cunningham that he was still a man.

Scout, Jem, and Dill see an angry mob gather around their father one night outside Tom Robinson’s cell.  They are concerned, because they fear violence.  Scout does not know what to do, but she knows Mr. Cunningham because she has talked to him before.  She feels that he is a reasonable man.  She decides the best thing to do is just try...

Atticus says that Scout reminded Mr. Cunningham that he was still a man.

Scout, Jem, and Dill see an angry mob gather around their father one night outside Tom Robinson’s cell.  They are concerned, because they fear violence.  Scout does not know what to do, but she knows Mr. Cunningham because she has talked to him before.  She feels that he is a reasonable man.  She decides the best thing to do is just try to strike up a conversation with him to distract him.

“Entailments are bad,” I was advising him, when I slowly awoke to the fact that I was addressing the entire aggregation. The men were all looking at me, some had their mouths half-open. Atticus had stopped poking at Jem: they were standing together beside Dill. (Ch. 15)

Mr. Cunninghman seems to notice Scout for the first time.  It wakes him up, and makes him realize where he is.  He stops being part of the mob, and becomes a man.  He realizes that he is Mr. Cunningham, father to Walter Cunningham, and he is about to attack Atticus Finch, father to Scout.

Atticus placed his fork beside his knife and pushed his plate aside. Mr. Cunningham’s basically a good man,” he said, “he just has his blind spots along with the rest of us.” (Ch. 15)

Atticus is aware that Mr. Cunningham is not a bad person.  He also does not think that Cunningham would have hurt him.  He might have hurt Tom Robinson.  Cunningham was acting the way he was because he was protecting Mayella Ewell.  He believed that Tom Robinson attacked her.  He did not approve of a black man attacking a white woman, obviously.

Atticus stood up for his client not just because he believed he was innocent, but because he believed he deserved a fair trial.  He knew that people might target him, and he was concerned that Tom might get hurt.  He risked person injury to protect his client.  He realized that when people get together in groups, they stop thinking as individuals and start thinking as mobs.  This is the blind spot he spoke of.