In To Kill a Mockingbird, by Harper Lee, what makes Jem decide to return to the Radley yard the night that Mr. Radley fires his gun?

In Harper Lee's To Kill a
Mockingbird, Jem, Scout and Dill go to the Radley house in Chapter Six so the
boys can look through the window to see Boo.

Scout is not
crazy about the idea, but Jem says she can go with them or leave. This is Dill's last
night in town, and she doesn't want to be left out, so she follows along. While Jem
looks through a window, there is a shadow on the porch which frightens all three of them
away. While they are running, they also hear a shotgun blast, which they believe is
aimed at them. To escape, they climb under the fence that borders on the school yard.
Jem lifts the fence for the others to climb under. Dill and Scout run ahead and notice
that Jem is no longer with them. Returning to the fence, they find that Jem's pants are
stuck, and his is kicking them off, leaving them there, and they continue to
run.

When the kids return to their house, the adults are
gathered outside investigating the commotion of the gun shot. Jem is without pants,
which Dill and he explain is the result of playing strip poker at the fish pool. The
adults are not happy with this information, but at least the youngsters were not
"involved" in the Radley incident (or so the adults believe). Atticus tells Jem to get
his pants back. The kids leave and split up.

Jem and Scout
go home, but Jem prepares to go out again. He tells Scout that he
must return to the fence. If Mr. Radley finds the pants still there
in the morning, the word will be out and Atticus will discover the truth of what they
were doing—after he warned them twice to leave the Radleys alone. Scout threatens to
tell if Jem leaves, but Jem explains that Atticus hasn't had to punish him for a long
time, and he wants it to remain that way.

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'Atticus ain't ever whipped me since I can
remember. I wanna keep it that
way.'

So Scout agrees to
remain silent, frightened and nervous that Jem will be shot by Mr. Radley when he tries
to get his pants. She seems to wait forever, but eventually Jem returns safely. He
quietly enters with his pants and climbs—still shaking—into his bed, eventually to fall
asleep.