Sorry to hear of this loss, Member. At least the queen was protected.
I've read of a very similar (near identical) situation. Dozens of bee butts in the air just as in your situation, right next to capped honey in the other case. Final determination after ruling out parasites and other factors was that they froze. I've attached a photo (with credit) of this case.
I believe (just my opinion) this is likely the culprit in your case as well for several reasons:
1) A significant number of bees were actively seeking food to (likely) activate their flight muscles to produce heat as they do to maintain hive temperature.
2) It appears (although we can't be certain) that the bees trying to eat died within a short period of one another--likely a temperature drop caused them to seek food. In leaving the nest core to retrieve food, they became exposed to cold. Similarly, the mass of bees on the bottom board were likely at some point the bees on the edge of the bee cluster, exposed to the cold, froze and fell down onto the bb. A parasite, while possible, wouldn't likely cause such a simultaneous death, but cold temps would.
3) The queen wasn't affected (as she likely would have been if it were a virus or parasite. They kept her warm at all costs, even sacrificing themselves in the process.
I don't know why they froze and other hives didn't--perhaps just lack of cold tolerance genetics for them...I don't know.
Were any of the brood or emerging bees dead in the their cells as well?
Photo credit: Ruth Rinehart (Source: Beekeeping Mentor in a Book, by Donald P. Studinski, p. 38, 2014)