The visions and accusations that Mary Warren mentions in Act II are due to the mass hysteria that she is being exposed to. I believe that she is "caught up in the moment" and is following what the other girls are doing at first. Once she begins to do this on a daily basis, she begins to think that she really sees or feels what she is pretending to see or feel. This is evident when we get to Act III, and Mary Warren can not prove to the judges nor can she explain to the judges why she can not faint at this point when, meanwhile, she had been pretending to do it for weeks prior.
Mary Warren seems particularly susceptible to others' influence, and the fact that the girls' claims are upheld by the court (and Hathorne and Danforth, both very powerful men) and confirmed by the confessions of some of the accused seems to convince Mary that there really are witches in Salem. Further, the sort of mob response she experiences when she's with the girls -- fainting when they faint, turning cold when they do -- seems to convince her that she's being influenced by something, and she attributes this influence to witchcraft, as the judges do, rather than the other girls.
Mary sees evidence where she's told it should be because she trusts the court's authority. She believes Sarah Good's confession. She believes that Goody Osburn cursed her for turning the old woman away hungry because she fell ill days after. Further, sitting in court, Mary says, "I feel a misty coldness climbin' up my back, and the skin on my skull begin to creep, and I feel a clamp around my neck and I cannot breathe air; and then -- entranced -- I hear a voice, a screamin' voice, and it were my voice -- and all at once I remembered everything she done to me!" Many in the court believe that Osburn is a witch, and so Mary begins to experience the symptoms she's learned are evidence of witchcraft. Her experience of these symptoms, created by her own mind, convinces her that Osburn and others really are witches, and so it continues.
I'm no expert, but I think Mary's motivation might come out of Mary not getting much attention.