I just went and looked. It says: The trouble is that the basic design of the hitches designed to attach to the rear axle is defective. It needs a bearing. as the trailer moves up and down, in relation to the bike, the axle bolt is turned. This does not seem to be a problem with eh Quick Release applications, and I can see why; but it is a problem when a bolt is used.

Again, the fix is that the hitch needs a bearing; however, that is hard to retrofit.