It's called "washboarding". I see my bees do it occasionally. The purpose is a bit of a mystery but sure looks to me like they are sweeping off the front porchI went to check out my bee after storm. all seem to make it had one knocked over. a couple of my hives are in almost rows moving back and forth all over front of hive. I still got bees going in and out. what is this dance and why are they doing it?