They swarm as part of their natural cycle of colony reproduction. They do it a lot. The hive will be producing a bunch of new queens, and when one of them is ready to hatch the incumbent queen leaves with about half the worker bees in search of a new home.

The new queen is born a day or two later and becomes the head of the old hive. She will go on mating flights for the next two weeks or so and will start laying after about 3 weeks.

HOWEVER! Bees dont just lay one new queen cell. They lay loads. So that new queen does one of two things, it either goes around the hive and actively kills the other unhatched queens be stinging them (Queens don't die when they sing), OR she decides to do a swarm of her own, taking half the bees of the already halved hive with her. In some cases this can become a bit of a chain reaction, with multiple swarms happening day after day (this happened to me last summer with one hive). The result is that the original hive becomes unviable and dies due to a lack of workers and no laying queen to replenish the work force.

As for bees leaving voluntarily, I believe it does happen if conditions in the hive are bad enough (i.e. Exposure). Though they will stay put through a lot of shit, to the point of the hive dying.