What is the brown stuff the bees are taking out?
Could it be they are taking out pollen to store honey?
doak
if its like dust they could be cleaning the bottom board from brood capping that have fallen, wish you could show a pic.
Bees have been known to eject pollen stores in favor of nectar. I haven't seen it happen this early often, it usually happens after a short dearth when all the bees could get was pollen and now with nectar coming in they need room for it. It normally happens in late August or Early September when the bees switch from hard growth to focusing on winter stores. The pollen is used much more in brood production than in bee survival, although they still require some.