I think she's feeling kinda down-on-her-luck, and then these rich white boys call her a bad name, and she figures this is a good way to get some fast cash.
IMO: Nifong is the main culprit here. I think the accuser came up with the rape story to avoid legal hassles. I doubt she had any intention of carrying through with it. She got trapped when Nifong decided to get involved.