The Boston Herald has now published a more detailed account of the fight, which brought about 100 players onto the field.
According to the Herald, Somerville police called for backup when things got ugly, and Massachusetts State Police responded to the melee.
A fight between two players apparently started after a play in which a Diva running back broke her leg. During a later play, the Militia got an interception, and it seems a member of the Diva got into a fight with a member of the Militia (see comments below).
When the fight broke out, the benches cleared and a "general scuffle ensured," the Herald reports.