Usually it's too many queen cells and a large enough population. It's happened to me several times. When I do splits I like to leave 2 queen cells instead of just one in case the one I picked was a dud, but if the hive is big they will sometimes swarm with one of those virgins. If a hive swarms or is split and there are many queens cells in the hive, instead of one of the queens stinging the others to death, the bees may swarm again, and again, and again, eventually swarming the colony to death as it were.