The incident that precipitated the Alarm was a British raid on the colonial powder stored at the Powder House on the Somerville/Medford line. It was one of several "alarms," including ones in Marblehead and Portsmouth, NH, that let the Provincials know the British were clamping down on them, militarily, before Lexington and Concord.
Ironically, not twenty years before, New Englanders had been instrumental in driving the Acadians from l'Acadie through sheer terror---and---grabbing their guns. In other words, they knew very well what could go down, because they'd been on the other side.