According to The Boston Globe, the deciding factor was an impassioned appeal from a Somerville police chief Robert Bradley. He appeared in commercials warning viewers about the trouble that would come should the ballot question pass. The campaign specifically targeted women since it was clear that men were already for the proposition. It's estimated that women voters saw the ad 11 to 15 times in the eight days before the election. Before the campaign, the proposition was expected to pass.
The agency responsible for the campaign, Rasky Baerlein Strategic Communications, specializes in public affairs. They filmed and had the ad on the air in about a day.
Now the Massachusetts Food Association, which supported the proposition, has filed a complaint with the State Ethics Committee, questioning whether Bradley filmed during work hours (a city spokesperson says no) and upset at his participation in the ad while wearing his uniform. Sour grapes?