This situation was on my mind a lot last night. I was thinking that maybe this happens due to a swarm gone wrong. I don't know what your weather has been like but around here, it's been rainy, overcast, and a little cooler than I would expect for this time of year. That said, I wonder if the hive produced a swarm cell, and the new queen emerged at a time when the weather was crummy, the hive though ready to head out for a new hive may have been held up due to the crummy weather and was waiting for a better opportunity to leave. This opportunity may have been so long in coming that they decided to heck with it. The new queen and the old queen may never have gotten into a battle royal because one or the other went way up into the hive and was tricky enough to not get wacked when coming to and from mating flights.
I maybe stretching it, but I'd be interested in some other thoughts.
Regardless, I am going to be splitting this hive and 4 others as soon as the weather breaks. We're supposed to be getting another long stretch of rain so I am not sure when this will get done.