Honey bees swarm when they run out of room in their location, if they have a new Queen and the old one decides to leave, if they are disturbed badly; there are lots of possible reasons. One of the largest swarms I have had to deal with recently was in a tree near where I live, about 25 foot up a hazel tree (not very easy to climb). The left hand photo here shows them before I tried to capture them. The right hand photo shows me holding one of the sections of the swarm.The photo below shows part of the swarm walking up the branch into the Nuc Box. They say there are about 8000 bees in a kilogram - this was a 1.8kg swarm, so about 15,000 bees. They were really gentle though; I had to keep going up and down the ladder to collect pockets of stragglers, and I didn't get stung at all. After I got them into the Nuc Box (a temporary hive for swarms) and the next day transferred them into an empty hive with some sugar solution to feed them and tempt them to stay. See more about catching swarms here.