"Atticus got up and walked across the porch into the shadows and his youthful step had returned.” What caused this change in To Kill a Mockingbird?

Halloween night had been a rough one for Atticus. He was
already tired from spending a week in Montgomery, and he decided to skip the school
pageant. Then he learned of the attack on his children. Atticus was frantic, concerned
about Jem's injury and his son's possible connection with Bob Ewell's death. It was one
of the few times in the novel that Atticus wasn't thinking clearly. Even though it was
Boo who brought Jem into the house, Atticus didn't initially think that Boo was
connected with the death: Atticus thought Jem had killed Bob, and it took a bit of
convincing from Sheriff Tate before he was convinced that Boo--and not Jem--had killed
Bob. Atticus then realized that Jem may have to testify in support of Boo, giving him
even more worries. But Sheriff Tate's decision to call Bob's death self-inflicted solved
the problem. Boo and Jem were off the hook, but Atticus still didn't like going along
with Tate's way of thinking. It was Scout who eased Atticus' mind, telling him that the
sheriff had made the right decision, keeping Boo from being dragged through a public
investigation.

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"Well, it'd be sort of like shootin' a
mockingbird, wouldn't it?"

It
was only then that Atticus' "youthful step" returned. Jem would recover from his
injuries, Scout was okay with the sheriff's secret, and none of them had to worry about
Bob Ewell again.