Why did Jem get so angry at Scout?

Scout finds Miss Gates hypocritical because she remembers that on the day of Tom's trial, she overheard Miss Gates say that she thought it was, "time somebody taught them a lesson, they thought they was getting' way above themselves, an' the next thing they think they can do is marry us." "Them" meant black people. In Scout's mind, this doesn't make sense and she goes to talk to Jem about it. Jem responds very angrily, and tells her he never wants to talk about anything having to do with that trial again. Scout is taken aback and goes to Atticus, who assures her that Jem just needs some time to think about things, and then he'll be himself again.