Mayella is much like another of the novel's characters who avoids the rest of society by remaining in his house. Like Boo Radley, Mayella is a social outcast within her own home. She is the lone adult female presence in the home, and she rarely has the opportunity to leave the house (except, probably, to buy the occasional groceries). She has the rest of the Ewell brood to look after while Bob prowls around Maycomb and the local swamps. So, her appearance on the witness stand in front of a crowded courtroom must have been just as unnerving to her as it would have been for Boo. Mayella seems to be an emotional wreck, and she begins crying long before Atticus begins his own round of questions. She has already witnessed Atticus's cross-examination of her father, and she tells Judge Taylor that she is scared. The judge lets her "cry for a while," and then he orders her to stop.
"That's enough now... All this is strange to you, I know, but you've nothing to be ashamed of and nothing to fear. What are you scared of?"
"... Him," she sobbed, pointing at Atticus.
She may realize that all of the Ewell family's dirty laundry will be exposed by Atticus's questions. Mayella must also be afraid about the story she has to tell on the stand. No one knows better that Tom is innocent of the charges, and she is merely following her father's story--that Tom has raped and beaten her. This is another fear: that her father may beat her again if she does not testify in a convincing manner. These fears, in addition to her own immaturity, "ignorance" and lack of "good sense," keep Mayella sobbing during most of her time on the stand.