If she's really gone missing then she is probably very afraid (although I wonder how solid the cablenews source on this is).
She probably fears either for her physical safety or that she risks going to jail. If she fears for her safety, perhaps Mangum's boyfriend and/or pimp has threatened her with death or injury unless she supports Mangum's story.
If she fears jail, then perhaps she's afraid that sworn statements she made to police supporting Mangum's allegations are false, and she could be sent down for perjury.
I very much doubt she is all that afraid of the Duke students or their parents, to the point where she would go in hiding. She probably doesn't have much in the way of assets anyway and is essentially "judgment-proof."
Listening to Kim's voice on the 911 call, she doesn't sound like she is from around there. It's possible that she left because she doesn't want her real identity to be revealed. Keep in mind that she is an escort, an illegal profession: she may have open warrants out on her or her family may not know what she does for a living.