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 Elizabeths or that she did not put the pin in it.