What might be the reason for Mayella's crying in the court?

At first, Mayella claims that she is afraid of Atticus. Likely, she was. In my opinion, Mayella cried because she was about to launch into a series of lies.... her tears may have reflected this. Even more likely, she is terrified of her father. Mayella sits in court a beaten young woman. She is immature, unloved, frightened, and willing to do just about anything to make people believe her. She is defensive with Atticus because she's never had any respect. Thus, she expects disrespect.