Ugh. 19 replies and not a single satisfactory answer. And some outright wrong ones. The reason behind the phenomenon is in how FP flash works. I'm lazy to explain it fully, but in FP mode, the flash fires multiple times to cover the entire frame when the shutter is only exposing part of the frame. What that means is that in situations where the flash is the only, main or significant source of light, you'll get disjointed trails of the action which show the areas where the flash lit up portions of the frame during the course of the 1/2000s (or whatever shutter speed during the FP sync).