That's probably because it reflects more light (appears brighter) at certain angles, these stand out against angles with less light visible to the viewer. Adding a few more lights would probably help.
What concerns me is that the blur isn't uniform, which shouldn't happen when the light intensity is constant. What happens if you make the animation slower and extend the mblur duration (1/2 x anim, 2 x mblur duration etc.), does it change the outcome does it look the same?