In "The Crucible" why does Mary Warren turn against John Proctor? What techniques are used in that scene?

At the end of Act 2 in "The Crucible", John Proctor threatens Mary Warren's life if she does not go to the court to testify to the judges that the girls have been lying. She is extremely hesitatant to go because she fears that Abigail and the girls will charge her with being a witch -- and this is exactly what begins to happen in Act 3. When Mary begins to tell the court the truth, the other girls, lead by Abigail, begin to mimic her every word; therefore, trying to show the judges that Mary is bewitching them. Instead of taking the fall herself, Mary goes against John by telling the court that he is a witch and was forcing her to tell the court that the girls were lying.