According to this bee expert, bees swarm for two reasons:
Though Queen Natasha has been quite prolific, the hive is not yet full. My girls are most certainly antsy due to the overcrowding resulting from recent nectar flows in town.
I’ve tried everything I can think of to keep them happily hived including the introduction of empty top bars into the brood nest. So far, they haven’t hived, but the youngsters keep building swarm cells.
Could it be an emergency preparedness tactic more than a full-on declaration of swarm?