Swarm on Sainte-Catherine

I was sitting at my desk yesterday (ok.it's more of a table) and heard something that sounded like a swarm of locusts. Well, i can only assume it sounded like a locust invasion, as i have never experienced one but have watched enough movies to have a vague idea (as if the filmmakers knew anyway). Wondering (and somewhat eager) whether the divine-being (whoever the fuck it is) was finally obliterating the human race, i poked my head out of the office and saw a swarm of skateboarders and a sprinkling of cyclists speed down Sainte-Catherine. I ran down to the corner as the tail end passed, and there were a couple cop cars with their sirens on dodging traffic to chase the group. Does anyone know what that was?