Everything in this hive was in great condition with no pests, a good amount of resources and plenty of good looking brood. A strong healthy colony with lots of bees. Moisture levels are normally very high inside a hive, around 70 to 90% humidity.
It looked like it just suddenly stopped with lots of dead bees everywhere. There was a large amount of rain and a few hot days. To me it looks like the hive could have overheated as it was exposed to long periods of direct sunlight and then high humidity levels started the mould.
Hive placement could be critical in some locations. Exposure to full sun for hours, close proximity to the ground with radiating heat and a roof could be factors to hives over heating. Try to keep hives under the shade of a tree and away from areas that radiate too much heat, provide a nice large roof.
