Why has Mary Warren disobeyed her employers and gone to Salem?

Mary Warren is also gaining a sense of power, as part of the court, that she has never had before.  As a simple servant, she has been at the mercy of the Proctor's, but now, as part of the court, she is the one wielding the power.  After all, when she returns and is nearly whipped by John, she tells them that Elizabeth has been accused of witch craft but she stood up for her good name.  Like the other young girls in the court, Mary is now getting power and attention that she never had in her life.