It is from the bees walking across the screen bottom board and the pollen gets knocked off or it may fall when they are walking on the frames or are putting in the cells. I watch my OB hive and see them dancing in the frames and then they go and find a cell to place the pollen in and they are very careful in placing it, putting their hind legs all the way into a cell and pushing the packs into the back of the cell. Then a house bee will head pack it in the cell. I have never seen them drop them during this. I use oil trays and most of the pollen drop is near the front of the hive where they are walking on the wire cloth. I do see lines of pollen that show me which frames the bees have brood on so they probably do drop it while on the frames.