I have a hive doing the same thing today. I don't have a clue what they're doing. Mind if I wait for an answer with you? 
The bees don't think it's a waste.What is up with this behavior? Seems like a wast of effort.
It is mentioned it the lit. Only speculation as to why. In my experience it appears to happen often in healthy colonies. Perhaps they are marking their turf?Washboarding. I don't think anyone really knows why they do it, but they normally do it when the flow is over.