One of the first causes is overcrowding. If they feel they have ran out of space, which they probably have if they are "boiling over", then they will swarm. Most time swarm cells are located on the bottom of the frames. I would move the old hive about 10 feet and set up a complete new box in the old spot. Take the frame with the queen cells, making sure you don't take the queen, with one other frame of sealed brood, and a couple frames of honey and pollen.All the field will return to the old location. the mother hive will do ok in the new location. Check back in 10 to 14 days for eggs/brood in the new hive. :) d2