My guess is that it's some kind of light spill from the saber glow element - that is, the film frame was just slightly larger than the opaque black Kodalith, so the animation stand backlight leaked around the edges. Then, during optical compositing, this backlight spill would have been burned into the darker, less exposed edges of the live-action element.
I believe this sort of "backlight spill" also occurred in the electricity animation when the Jawas knock R2 out. It was brought up in the thread for DJ's project - here's the picture that was posted there:
