This single-layer OH has a shallow over three deeps. It is in mid-MD. Yesterday Feb 16, 2020 was overcast and 43 degrees. Bottom two deeps are filled on both sides with capped brood! Top deep has brood in all stages. Queen present and laying. Top shallow comb completely empty, but bees are keeping SHB confined to the cells in the shallow frame. Hive is very populous. But yesterday, a large cluster formed on the outside wall despite it being pretty chilly. Also, the bees would gather in a beard on the tip of the right-angle PVC (the elbow prevents prevailing winds from blasting straight into the observation hive entrance. Some photos show it removed because I was doing my inspection) and then the blob of bees became too heavy and dropped to the ground. I picked up a handful of bees that appeared dead but were only in torpor. They started walking around inside my cupped hands once they had warmed up. No signs of DWV. No twitching or other signs of Chronic Bee Paralysis Virus. No smell of dead bees on the ground. They were only in torpor but not dead. After a while most of the bees returned to the hive. Many, though, remained clinging to the outside wall where they are stuck until it warms up. Hopefully later on today.
Do you know what might have caused such a large number of bees to leave the hive on a cold, overcast day and cling to the outside wall?