When the bees are washboarding they are cleaning/smoothing the surface and applying pheromones to the surface. Why they wait until after the nectar flow winds down I am not sure. Perhaps all the activity of incoming foragers brings debris and odors that they wish to remove and they only want to do the job once.
Maybe so and maybe they are outside waiting for returning recon flyers, hoping some incomers will treat them to a waggle dance, showing the way to more nectar? Kind of like flyers on the flat top with the engines revved up, anxiously awaiting to be flagged for takeoff? lol