or perhaps an anticipated strong honey flow, the worker bees (all females) feed the eggs differently, which create Queens instead of more workers. Once the new queens are about to hatch, the old queen leaves with a bunch of bees, forming a swarm. The swarm typically lands on a branch or structure near the original colony, where scout bees are sent out looking for a new place to live (a hollow tree, or a hollow wall in your house). The scout bees return, communicate to the swarm, then they all take off and fly together to their new home.

I intercept the swarm when they are sitting on the branch. Typically they are pretty happy to get a new house so easily.

On this particular location, this was the third swarm we caught, which means that the original colony (it was in a huge oak tree nearby - we could see it) made multiple queens and swarms (sometimes once the first old queen takes off, the colony only allows one queen to survive, so there is only a single swarm versus multiple ones like this one).

Swarms are very docile. They aren't defending anything. You can literally stick your bare hand in one (I have done that personally).

This kind of thing happens everywhere there are honeybees, both captive or those that have taken up residence in the woods.