Washboarding bees

What does it mean when bees rock back and forth at the front of the hive?

One of our hives has had a lot of bees washboarding on the front. All the other hives don't display this behaviour, just this one. And nobody has quite figured out why bees do this.

It just shows you that nature still has it all over us mere humans. Rather beautiful though, don't you think?

(The noise in the background is the ute running, not bees humming. Drat!)