What is the suspenseful moment in chapter 6?

Jem and Dill want to sneak over to the Radley place and peek into one of their windows. Scout doesn't want them to do it, but Jem accuses her of being girlish, an insult she can't bear, and she goes along with it. They sneak under a wire fence and go through a gate. At the window, Scout and Jem hoist Dill up to peek in the window. Dill sees nothing, only curtains and a small faraway light. The boys want to try a back window instead, despite Scout's pleas to leave. As Jem is raising his head to look in, the shadow of a man appears and crosses over him. As soon as it's gone, the three children run as fast as they can back home, but Jem loses his pants in the gate. As they run, they hear a shotgun sound somewhere behind them.