This happens when you are exceeding the sync speed and only part of the sensor is exposed during the flash of light.Try reducing the speed to something below 1/250th and see what happens (you might need to slow down things even more...).
----frame4zoom for Nikonians
Copyright © Nikonians 2000, 2013
All Rights Reserved
Nikonians®, NikoScope® and NikoniansAcademy™ are trademarks owned by Nikonians.org.
Nikon®, Nikonos® and Nikkor® are registered trademarks of Nikon Corporation.