This scene comes after the madness that has started in Act One, and as Mary Warren returns to the Proctor household where she works, she tells John and Elizabeth more about how the madness is sweeping through Salem and more are being caught up in it. We are informed that the number of accused people now reaches 39 and that Goody Osburn will hang. It is when Mary protests that she is doing "weighty work" and that she must go every day to the court to do her "work", that John Proctor threatens to whip her. It is only when John Proctor is about to whip her that Mary Warren confesses that she saved Elizabeth Proctor's life. Note how she replies to Elizabeth's wondering question:
Somewhat mentioned. but I has I never see no sign you ever sent your spirit out to hurt no one, and seeing I do live so closely with you, they dismissed it.
Of course, Mary is "bound" not to tell who accused Elizabeth Proctor, but from what we know of Abigail and her past relationship with John Proctor, and the fact that we also know that she drank a potion to kill Elizabeth Proctor, we can only surmise that this accusation emerges from her in an attempt to gain the man she loves.