Beyond the basic answer that she was attracted to him, there ay be a number of reasons for Mayella's behavior towards Tom. She obviously thinks her word is more trustworthy than his simply because she is white. She thinks she can break the rules of her class and culture by making advances towards a black man.
Despite her poverty and lack of education she thinks she is superior to this honest man, and never stops to think that he might reject her. Her life is empty and dull and she seeks amusement and a sense of power. By trying to manipulate and dominate Tom she is trying to raise her own sense of self-worth. But this attempt fails, and in her humiliation she retaliates for Tom's rejection by accusing him of rape.