After Mary Warren is asked to pretend to faint in court to prove that she was also faking this when she originally testified in court, and she cannot, Danforth and Parris are convinced that she is being coerced into telling this "new truth" to the court by John Proctor.
Danforth and Parris are reluctant to believe Mary Warren because they are suspicious of her motives for suddenly coming into the court to tell the supposed truth.
Danforth and Parris suspect that because Elizabeth Proctor has been arrested, that Mary Warren is there because John Proctor has forced her into it. They believe that Proctor has brought Mary Warren to court against her will and is pushing her to tell the court that the girls are making up the story about witchcraft to save his wife's life.
Afterall, the poppet at the heart of the arrest of Elizabeth Proctor was given to her by Mary Warren. So, if Mary Warren really put the needle in the doll's belly, then Elizabeth is innocent. It seems too convenient that Mary Warren would confess that the girls are all faking at this moment when Elizabeth has been accused of witchcraft by way of the poppet against Abigail Williams.
Danforth and Parris are suspicious and they push Mary Warren, reminding her that she will hang if she lied to the court before, she breaks down and takes back her new truth and turns on John Proctor, accusing him of forcing and threatening her and she accuses him of cavorting with the devil.