I was shooting a pano this morning and many of the images had the lights with streaks to the right. Always the right. I was panning to the right, manually rotating the camera, releasing it and pressing a remote release with a one second delay. The lights in the images often appeared as: O-----
I am guessing it is not the camera itself (Nikon D810), so it has to be the tripod or its head. The head is a Vanguard SBH 300 on Vanguard Alta Pro 263AT legs. It's pretty solid and stable. If it were ground shake, I'd expect the light line ("-----") to wiggle and go in different directions. It is perfectly straight and always to the right. I was always turning to the right, too. Next time, I'll go in both directions.
But what could cause it? (No, I didn't touch the tripod. That's why I used a remote release.)