Sometimes there will be a swarm when the main colony is threatened with disease, mites, spraying, etc. I do a visual check on all new swarms, as well as use only my own used equipment or new....to keep the chance of bringing disease into my hives.
I have also been keeping any new swarms in a different location to lessen the chances of my home bee yard being exposed to anything carried in by the new bees.
It could be possible that the main colony had disease, so they swarmed.
Brenda