I bought a 5 frame nuc 2 weeks ago and put them in a 10 frame hive. There was no queen but several queen cells. (never buy from that man again) but anyways when I was on my way to check them this afternoon they swarmed. Why? They had plenty of room. Was it because they might have been about to swarm on the man that sold them and they were just programed to do it? well, I was able to get the swarm in a nuc with a frame of brood and put a hivetop feeder on them. That was a rush when I shook the branch and tey fell into the box and had thousands of bees flying around me. Set the box in the crok of the tree limb and the rest marched right in. But why did they swarm?