why does jem react violenty when scouts talks about what she over heard at the court house

Jem is crying and angry - he thought that the case was clearly in Tom's favor. Atticus is exhausted and when Jem asks him how the jury could have done it he responds, "I don't know, but they did it. They've done it before and they did it tonight and they'll do it again and when they do it - seems like only children weep."