Here in the Midwest it happens some years quite bit. Late season cold snaps. Warm spell in Feb or early march, lots of pollen and brood made, then 4-5 days of freezing or sub freezing temps. Bees have to choose, keep brood warm, or move to food. They always choose brood. Bigger the hive more likely. Usually take 4 plus days of cold to cause it.
Charles
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