My guess would be pesticide. Just by the action of the bees and the sudden turn of events. The bees should be active on the comb unless they are sick form something like pesticide. It’s a real bummer when some careless farmer sprays without thinking about what they are killing. Then there are a lot of people that look at all bugs the same, way with a kill’em all attitude. Honeybee’s or cockroaches are all the same to them.
The Busy Bee teaches two lessons: One is not to be idle and the other is not to get stung.