OK, I have planted sunflowers but I have never seen bees on them like that. What is so special about those plants? Did someone spray them with a bee pheromone?
That would be an easy swarm to capture. I was never that lucky. I understand that bees go to a waiting area when they swarm and wait for the scouts to come back with a suitable nesting spot and then leave again. I guess they might as well wait there as anywhere.
It very easily could have been staged, by tying on a caged queen and then turning a bunch of workers loose, the same way they do "bee beards" on people.