why does mary warren change her testimony and join abigail and the other girls?

When Abby rails against the judges and insists that questioning her is insulting, Abby begins to act as if Mary Warren is "witching" her. Mary is so caught up in the girls' behavior and she knows they will not stop until they have "defeated" her. She knows that when Abby is the ringleader, Abby will get her way. So Mary goes to the side of the girls and accuses John Proctor of having forced her to sign the devil's book.