Atticus gives the reader his reasons during his summation to the jury. Atticus says he pities Mayella, but not to the extent of allowing an innocent man to go to prison for her own guilty conscience.
"I say guilt, gentlemen, because it was guilt that motivated her.
Atticus claims that because she is white, she knew the "enormity" of her actions--that of tempting a black man. Her guilt was so strong that her only recourse was to place the blame on the man she tried to entice.
"She must destroy the evidence of her offense...
She did something that in our society is unspeakable: She kissed a Negro... No code mattered to her before she broke it, but it came crashing down on her afterwards."
Atticus also believes that her father, Bob Ewell, had a hand in the accusations. He reminds the jury that it was he who swore out the warrant, and he reminds them of Ewell's "conduct on the stand."k