In The Crucible, why has Mary Warren disobeyed her employers and gone to Salem?

Mary says she has to go to Salem because she is part of the court proceedings.

Proctor is very upset about Mary Warren going to Salem without his permission.  Mary Warren is the Proctors' servant.  She is involved in the court proceedings for the Salem Witch Trials.  Proctor does not want her to have anything to do with the trials, because he is opposed to the trials completely.  He knows that there is nothing to them except vengeance and persecution.

How dare you go to Salem when I forbid it! Do you mock me? I‘ll whip you if you dare leave this house again! (Act 2)

Mary Warren complains that she does not feel well, and being in the proceedings all day is making her sick.  Proctor complains that she should not be at the court when she has responsibilities in his home.  She is in court all day and therefore she is not doing the job he pays her for.

Proctor also notes that Elizabeth is not feeling well.  It later transpires that she is actually pregnant, but has just become pregnant and is not showing yet.

Mary gives Elizabeth the gift of a doll she made during the court session.

MARY: (Crossing to Elizabeth, taking a small rag doll from pocket in her undershirt.) I made a gift for you today, Goody Proctor. I had to sit long hours in a chair, and passed the time with sewing. (Act 2)

Elizabeth tells her it is a “fair” gift, which is high praise for a Puritan.  Mary tells her that they all need to love each other.  That is fairly ironic, since there is no love going on.  People are just pointing fingers and accusing others to get back at them or get something they want from them.

The doll turns out to be important, because it is presented as evidence that Elizabeth has been using it for witchcraft.  A pin is found in the doll and everyone thinks that it is some sort of voodoo doll.  They do not believe that it is not Elizabeth’s or that she did not put the pin in it.