A wasp swarm refers to either a large group of wasps concentrated around a tall structure during mating season or the mass of wasps that rush out of the nest to defend it when disturbed. Wasps do not swarm for any other reason.
Unlike honeybees, which swarm by following their queen around to collect more members for the colony, wasps don't typically travel in groups. During the cooler months of fall, the males claim spots high off the ground, secreting pheromones to attract a future queen. It is common to see a swarm around the neighborhood's tallest chimney or the water tower. Nests can vary drastically in size, from holding 20 wasps to thousands. Wasps are usually non-aggressive and eat a lot of pests and insects.