Response of Jem and Atticus explanation to his behavior:
When Scout asks about the Trial to Jem, he explodes and shout at Scout angrily. Jem asks Scout not to mention about the courthouse to him again. Scout was confused and asks Atticus why Jem is acting like this.
Atticus says that Jem cannot accept the outcome of the Trial. He is still trying to prove his innocence and he would need some time to come to terms with the way things went on the Trial.