This is an excellent question and there are no specific answers to be found in the course of the novel. Braxton Bragg Underwood (who has been named after one of the Confederacy's most inept generals by his father "in a fey fit of humor") is the editor and owner of the local paper, The Maycomb Tribune; he rarely leaves his office, preferring people to bring their news to him. By his side is an "ever-present gallon jug of cherry wine." Since the rest of the town already knows that Tom Robinson is going to be transferred to the county jail, Underwood must have also heard the news. And since the newspaper office is located next door to the jail, it would not have been unusual for Underwood to be keeping an eye out for any possible trouble that may occur. He and Atticus are friends--Scout had seen them together at the church earlier that day--so Underwood may have decided to play the silent sentry when the lynch mob came to take Tom. Atticus himself is puzzled by Underwood's appearance and his defense of Tom--
"You know, it's a funny thing about Braxton," said Atticus. "He despises Negroes, won't have one near him."
But above all, Underwood believes in printing the truth, and after Tom is killed at the prison, he laments Tom's death, comparing it to
... the senseless slaughter of songbirds.
Underwood may hate African American, but he does believe in justice. He wants to see Tom tried legally (perhaps in part for the story it will present him), not hanged by a lynch mob; and he must realize after the verdict that the jury has come to the wrong conclusion. Underwood may be a racist, but he is an honest one, and he has a moral obligation to the town to present the truth to his readers.