Canon 5D II, 20-35 f/2.8L, 28-80 f/2.8L, 70-200 f/4L IS, 100-300 f/5.6L, 100 f/2.8 Macro
Again, without knowing what the other photographers were using, their stobes and what modifiers they were using, what the ambient light was (spot light on performer?) its hard to be certain exactly what's going on.
But its likely explained by the first scenario in my reply to Silvermoon in post #19.
The right side (darker part) was your ambient exposure (without flash). The left side was lit with another photographer's flash + ambient light.
There's no reason to expect your shutter actuation to be in sync with another photographer's flash so part of the frame is block by your shutter during its opening/closing movement, similar to an out of sync flash exposure above max flash sync speeds, except there's ambient exposure present in your shot.
This isn't an illustration of:
if you're determined to get an explanation, I'd suggest asking this in the 'Photographic science and technology' forum in dpreview. There's quite a number of scientific minded posters there that I think can give you a definitive answer.
When I saw this, it reminded me of this thread. Perhaps it's a similar issue?
Hi it looks like shadow cast on the filter from the hood