Mayella sees Atticus as the enemy. She most likely expects her to be mean and harsh with her while she is testifying. We are also left to assume that she is trying to protect her father from being charged with abuse, so she is maintaining the lie that Tom Robinson beat her. Atticus does not speak harshly to her, though. He speaks to her with more respect and sincerity than she has ever been exposed to before. We know this from her response -- she thinks he is being sarcastic at first. This fact, combined with his questioning about her father's treatment of her and whither or not she has any friends, takes Mayella through a process of apprehension, confusion, then hatred of Atticus.
Mayella doesn't trust Atticus. He's accused her of kissing a black man, which is completely counterculture at that time, but - as someone with a learning disability- this was the only way that she thought she could have any kind of relationship at all with a man, and she was doing everything she could do to maintain any kind of dignity she had left.
Plus, because Mayella was being groomed to commit perjury on behalf of her father, she was likely being prepared by him to view Atticus as the bad guy, and as someone who was trying to set her up if she wasn't careful.