In chapter 15, the children follow Atticus and find him sitting outside of Tom Robinson's cell. Suddenly, the Old Sarum bunch arrives to lynch Tom Robinson and encircles Atticus, who refuses to move out of their way. Scout is unaware of the dangerous situation and instinctively runs into the middle of the group of men. The men are astonished at Scout's presence, and Jem and Dill proceed to follow her out of their hiding spot. Scout immediately recognizes Mr. Cunningham and attempts to have a casual conversation with him. Fortunately, Walter Cunningham sympathizes with Atticus's difficult situation and instructs the mob to leave the scene without harming anyone.
Later that night, Scout crawls into bed and realizes for the first time that she and her family were in an extremely dangerous situation. The entire time that they were surrounded by the mob outside of the jailhouse, Scout was unaware that the drunken men wanted to lynch Tom Robinson and were willing to harm Atticus in order to kill his client. Scout says,
The full meaning of the night’s events hit me and I began crying. Jem was awfully nice about it: for once he didn’t remind me that people nearly nine years old didn’t do things like that (Lee, 157).
Essentially, Scout cries because she is overwhelmed when she recalls the dangerous, traumatic situation that she narrowly escaped.