This is certainly an opinion question but it can be proven by facts in the story. At first, when Mary Warren went to testify, she was indeed under the fear, although a healthy fear, of her boss. Thus, she was ready to testify in his favor.
After some time in the courtroom with the girls pressuring her to conform and the judges pressuring her to perform (she had claimed that what they were doing was pretense or pretend and they wanted her to pretend right in front of them to prove it) she caved.
What made her cave completely and exactly was the moment when Abby made it seem as if Mary Warren were bewitched. After some mockery from the girls, Mary knew if she didn't join them, they would have her look to be the very next hung witch in the town.
Initially, John Proctor brings Mary Warren to the court to confess that she and the other girls were lying when they accused people in Salem of witchcraft and that they were pretending when they fainted and seemed to freeze in the courtroom in the presence of those accused. First, the judges ask her to pretend to faint again, alone. She cannot. Then, when Danforth begins to question Abigail's integrity as a result of Mary's story, Abigail threatens him and pretends to feel "a cold wind," looking straight at Mary. The other girls join her, teeth chattering, accusing Mary of sending a "shadow" on them. Mary panics and tries to run.
Soon after, Abigail accuses Mary of sending out her spirit as a "yellow bird" who "want[s] to tear [Abigail's] face." All the other girls pretend to see this yellow bird as well, and, as the tension rises, they begin, frantically, to repeat everything Mary says, as though she has bewitched them all. Finally, knowing that Abigail is laying the foundation to accuse her of witchcraft, Mary turns on John in an effort to save herself. It is obvious that Danforth thinks Proctor is a liar, and Mary takes the opportunity to accuse him, knowing that she will be believed, just as Abigail knew that Abigail would be when she initially accused Tituba, a slave, of witchcraft.