delber, it seems that once bees make up their mind to swarm - sometimes you can't stop them. If they were feeling cramped due to only one hive body, they might have already decided "We are running out of room." and even though you "built on", their minds were already made up.

I haven't heard of any swarms in our area yet.

I did put out two swarm traps near my two hives if they decide to swarm. And I'm hoping to get several more swarm traps placed around where I know feral colonies are.