Honey bees swarm when they leave their hive to establish a new one.
The main reason for this is overcrowding, which means the hive has become too small for the population or the queen has no room to lay eggs. Here in the UK this usually takes place in the Spring around May/early June.
Although it is possible to see occasional swarms later in the summer. Other reasons can include lack of food or water, disease or parasite infestation, pesticide use, human or animal disturbance, weather changes, or poor ventilation. Swarming is a natural response to these conditions and a way of reproducing or increasing the population.
During swarming, the colony divides into two. The old queen and about half of the worker bees leave the hive to find a new home, while the remaining bees stay behind with a new queen. The swarm of bees will typically rest on a nearby structure, such as a tree branch or a fence, while scout bees search for a suitable new location for the colony. Once a new location is found, the swarm will move to the new site and establish a new hive.
Swarming is an important part of the honey bee life cycle and helps ensure the survival of the colony. If you ever come across a swarm of bees, it's best to keep your distance and contact a local beekeeper for assistance.
Many beekeepers, including myself, who are local to where the swarm is, are often able to collect a swarm or if not will know who you should contact.