What was the most likely reason that Jem continued to defy Atticus's requests for him to take Scout and Dill and leave?

In
,gets a "feeling" that he needs to head downtown, so the kids all head for 's office.
When they get there, they realize Atticus isn't at work, so they decide he could be visiting Mr.
Underwood. To reach his office, they must pass by the jail, and that's where they find
Atticus.

They arrive unexpectedly and are not noticed, at first, by the mob
gathered outside the jail. Atticus sits outside Tom's cell, quietly reading the paper, when the
mob confronts him. They are there to lynch Tom, and they order Atticus out of the way; he
refuses.

It is around this point thatrealizes that Atticus is in trouble and
bursts onto the scene, followed by Jem and Dill. Atticus clearly recognizes the trouble this
could cause for his children and asks Jem four times to take Scout and Dill home.

Jem squares up to Atticus and refuses.

Scout knows this is
significant (the two children obey Atticus almost always):

I...felt Jem had his own reasons for doing as he did, in view of his prospects once
Atticus did get him home.

Jem, a bit older than Scout,
realizes that Atticus is not going to simply stand aside and allow the lynch mob to take Tom
Robinson. He is a man of honor and justice, and this is the man he will defend...to the death,
if necessary.

As the second-in-charge man of the house, Jem likely determines
that he can help Atticus with the crowd if things turn violent, or he is hoping that the crowd
won't resort to violence with Atticus's own children standing present.

Luckily, Scout manages to salvage the situation by reaching out in very human terms to
Mr. Cunningham, and the near disastrous confrontation melts away, at least for that
night.