Most 'feral' swarms are escapees from managed hives. The closer you are to commercial apiaries and to the high density of hobby beekeepers in the Puget Sound area, the more likely you are to capture another beekeeper's swarm. Free bees is good, right? Not always. If the beekeeper was not managing hives well, feeding if needed, and treating for mites, those free bees may not be a reproductive swarm but may be absconding from unsurvivable conditions: hunger or Varroa levels so high that the hive cannot raise brood. "No food, no brood" means no hope of survival if they stayed where they were. "Hunger swarms", as they are sometimes called, or absconding due to Varroa parasitism can happen at any time of year. Some beekeepers think of them as summer and fall phenomena but rainy springs in Western WA can cause hunger swarming when a newly installed colony realizes it is in a "no food, no brood" situation with not enough accessible food to survive.
And of course, tendency to swarm genetics could play a role in the colony's decision to die in place or leave in hope of survival. What beekeeper always knows what is in the mind of bees?