I have a geologist friend and his reply was “Fuck knows”. I don’t have much of a chance of answering this myself. He states one theory, this is that the motion outer core shifts every Xnumber of years, this could be dictated by a secondary cycle in the convection currents dictating the movements of materials in the core. But as I said, theory. He says it’s just one of the (many) things we don’t yet know. I’ll tell him to find out for you. Life’s work and all that.