I had the same thing happen once, years ago. One of my hives swarmed while I was at work and I was able to collect the swarm and put them into an empty hive. Then, a couple days later there was another swarm, much smaller, in another place in the yard. I collected the new swarm and dumped them into the hive with the other swarm, since I had no other empty hives. The next day, there was a dead queen right in front of that hive. I figured both swarms had a queen and the bees probably killed the virgin queen with the small swarm.
I have read that a strong hive can throw several swarms.