Mayella may be nearly 20 years old and facing the unwanted responsibility of being the female head of the Ewell family, but she is friendless, "ignorant" and just plain scared when Atticus "rained questions on her." Mayella had already seen Atticus embarrass her father on the witness stand and, despite her immaturity, she must have known that the same fate also awaited her. Mayella had apparently never been treated with respect by any man before: Her own father beat her, and she mistook Atticus's honest respect as mockery. Nobody had ever called her "ma'am" before, and she believed that Atticus's "ma'amin' and Miss Mayellerin' don't come to nothin'..." She considered every man in the courtroom as "yellow, stinkin' cowards... the lot of you." Also, her own guilt must have been overwhelming, and by the end of her testimony, Atticus had gotten her to change and even contradict her own prior statements. In Mayella's confused eyes, Atticus was the enemy, the only man capable of exposing her lies and her "unspeakable" crime: that "She tempted a Negro."
Mayella proves herself to be ignorant of quite a few things on the stand. However, she knows she doesn't know things, and it makes her feel lost and easily offended.
Atticus, in trying to be a gentleman, treats Mayella in a way she isn't used to and probably doesn't think she deserves. By using manners, Atticus offends Mayella because she thinks he is making fun of her.
This scene paints Mayella as even more of a victim of her cruel father and often readers are feel pity for her.