In Act III, when Mary Warren is brought by Proctor to testify against Abigail and the other girls, they are quick to act to attack Mary and also to defend their own position. When pressurised, Abigail feigns a visitation of another spirit that she claims is Mary Warren, and which she and all the girls react to. Once Abigail has started to freeze, the others quickly follow suit, accusing Mary of being the cause. Note what Mercy Lewis says to her:
Mary, do you send this shadow on me?
Mary as a result crumbles and tries to flee the courtroom. She is clearly terrified of the influence that Abigail has over the girls and how they have turned against her. It is this that causes her to rejoin Abigail and the girls and go against what she told Proctor before. Mary is confused by the way the girls deliberately turn on her and break her.