At a guess, because crows are smart, resourceful little chaos goblins, with basically no fear, especially in groups. Maybe they like the noise the breaking glass makes, or maybe they're just curious to see what will happen. It wouldn't surprise me to learn that they've made a game of it, to see which of them can hit nearest the centre, or from highest up. I'm actually surprised that a single falcon manages to scare them off, after seeing what they do to buzzards round here. Even if the buzzard is minding it's own business, multiple crows will attack it in flight, dive it bombing and harrying it until it has left the area.