A lot is changing for the Finch family in To Kill a Mockingbird. Jem is growing moody and even unpleasant. Aunt Alexander has come to stay, and does not share Scout's outlook on life. As Scout reflects in chapter thirteen:
Aunt Alexandra was of the opinion, obliquely expressed, that the longer a family had been squatting on one patch of land the finer it was.
Aunt Alexander is very critical of Scout and her tomboyishness, and even persuades Atticus to berate his children for improper behavior. Scout is not prone to crying but this makes her cry, more from her confusion because she would never expect Atticus to worry about keeping up appearances. She has always thought that as long as people "did the best they could with the sense they had" that that would be good enough.
In chapter fifteen, when Atticus goes out one evening in his car (which is an unusual occurrence), the children follow him because Jem is worried about him. They see that he is sitting in front of the jail, and Scout runs to him interrupting a volatile situation unintentionally. The children become part of the interaction, and Scout fortunately defuses the situation without even being aware of it. Back at home, she is able to process "the full meaning of the night's events" (chapter sixteen), and realizes that Atticus' calm demeanor had hidden any real fear. This is when she begins to cry, as she realizes that her father had been in grave danger.