Mary Warren continues to accuse others of witchcraft for several reasons, but the primary motivation is fear. Abigail is very charismatic and has threatened the girls if they turn against her. Mary has witnessed firsthand what happens to those whom Abigail views as threats--they are accused. If Mary separates herself from the girls, she knows that they will accuse her of witchcraft and that she will end up in jail or sentenced to death.
A lesser motivation for Mary's accusations is the attention that she and the other accusers have received. Keeping in mind that Puritan girls had few rights and little attention, the trials would have been the most exciting event for many of these girls. Instead of performing tedious tasks around the Proctor home all day, Mary gets to go to town, sit in court, and, in essence, hold a position of power.
Finally, she is flexing her independence. As a servant to the Proctors, Mary is not her own person. She must do as they command. In going to town, she disobeys the Proctors, and there is little they can do to stop her.