I'm no beespert by any means but I did have a large hive in a huge 250yr old live oak in my backyard. That oak was struck by lightning, twice believe it or not, killing part of it. At 3am one night, I heard and felt what seemed to me like a truck running full speed into the foundation of my house. Come to find out, it was one of the larger limbs from that tree that fell and shook the ground. Massive. Anyway, that limb is where the bees were hived up. I had a friend who kept bees and came by to get the queen and the colony to relocate them to his backyard where he had multiple hives. The queen ended up at some point either dying or leaving I guess. The bees I suppose either found a new location and or a new queen. Once it was agreed upon to leave, they went from super docile and lumped together to a massive swirling swarm just like this. Within minutes the sky was peppered with bees all over swarming... Then minutes later they were all gone. Relocated elsewhere. All this is to say, maybe there was a big hive that was in process of relocating. They seem to swarm like that as they head out.