I recently had a small mating nuc with some just capped cells tear them all down a day after they capped them. It was only a frame and a feeder - from a larger queenless hive. Why would they do that? The entrance was closed with screen to prevent robbing until the cells were near hatching, so no usurpation would have taken place, nor mating if a loose queen was in there. They had plenty of food and pollen.