Have you tried setting your kit up in exactly the same way and then taking a shot of a blank surface such as a wall? If the fault is there again, then it is something in the camera or GND, if not, then it was something to do with the direction of light and how it was passing through lens or your GND.
It could also be just a natural phenomenon of the lower light reflections from under the cloud layer on the left, even though it looks like a straight line of darker pixels, reflections can sometimes do this kind of weird thing.
But as I say, if you can replicate under controlled conditions, then something is a amiss with your kit, if you can't then it was just the light on that particular day or something.
Dave