Because of Atticus's strong defense of Tom Robinson, the all-white jury takes a surprisingly long time—hours—to determine that he is guilty. This gives Scout time to think. She remembers what Jem once told her:
I remembered something Jem had once explained to me when he went through a brief period of psychical research: he said if enough people—a stadium full, maybe—were to concentrate on one thing, such as setting a tree afire in the woods, that the tree would ignite of its own accord.
This aligns with the idea that, contrary to Scout's wishes, the white community in Maycomb is concentrating on the need to keep Black people oppressed at all costs. Scout wishes, in contrast, that people would concentrate on Robinson being acquitted.
More significantly, the stillness in the packed courthouse as the spectators wait brings back Scout's memories of the silent streets when the rabid dog Tim Johnson was wandering free:
When the mockingbirds were still, and the carpenters had stopped hammering on Miss Maudie’s new house, and every wood door in the neighborhood was shut as tight as the doors of the Radley Place
This memory shows that people are as fearful of Tom Robinson being acquitted—or convicted—as they are of a mad dog on the loose. The trial has brought up the uneasy tensions that exist between Black and white people in the Maycomb community and white people's deep-seated fears of disruption of a social order that has worked comfortably for them—but now might "bite" them.
It's significant, too, that Scout remembers the Radley place at this moment, along with the stillness of the mockingbirds, because Boo is the other wronged innocent in the story—the other mockingbird.
This pause shows Scout maturing, processing events, and assessing the present moment in the light of her past experiences.