In what way has Mr. Underwood recently helped Atticus? Why is this behavior surprising?
Mr. Underwood is "the sole owner, editor, and printer" of the Maycomb Tribune (To Kill a Mockingbird, Chapter 15). The newspaper office is located right next to the jail in downtown Maycomb. On the night that the mob approaches, Scout, Jem, and Dill think that Atticus is alone outside the jail, trying to protect Tom Robinson. The children refuse to leave, fearing for Atticus' safety. They later find out that all along, "Mr. Underwood and a double-barreled shotgun were leaning out his window above The Maycomb Tribune office."
The next morning, Aunt Alexandra finds out that the children had sneaked out of the house to find Atticus outside the jail. She thinks that this sort of behavior is disgraceful. Aunt Alexandra is very proper, and she believes that children should be in bed at night and not outside, roaming the streets.
Atticus thinks that his children helped disperse the mob, but Aunt Alexandra thinks that their help was unnecessary because Mr. Underwood was there with his gun. Atticus notes that it is strange how Mr. Underwood was willing to defend him from the mob, even though he was protecting Tom, who is black:
"You know, it's a funny thing about Braxton," said Atticus. "He despises Negroes, won't have one near him."
Even though Mr. Underwood most likely thinks Tom is guilty, he is still willing to defend Atticus as he protects the man. Atticus finds this strange and surprising. Calpurnia is nearby when Atticus says this. Aunt Alexandra is surprised that Atticus would say such a thing in front of Calpurnia, who is black. Atticus says that Calpurnia already knows it, and that he will not hide anything from her.