Ok, well hopefully you have delayed them long enough with OSBN until the Main Flow starts.
Maybe something did happen to the queen in that hive with the emergency cells.
I've also noticed a few times in hives that have swarmed that there were older Queen Cells which looked like supercedure cells, then several younger swarm cells along the bottom. As if it was initially queen replacement and then it's like: "Since we are making Queens, we may as well take this opportunity to Swarm as well".
So maybe low queen pheromone concentration (due to increasing population) can trigger an initial supercedure response and then it turns into swarm preparation because of ideal seasonal conditions.