Nikon F100 shot through a Nikkor 28-70mm f/3.5-4.5.

Ah, crap. I've noticed that the rear lens element (the part that slides in and out with adjustment the zoom ring) is a little "loose" on the wide end of the zoom and it jiggles a bit inside the lens. Since I was shooting these inside the cramped quarters of a semi-abandoned building, all of the photos that I took on the problem roll were at or around the wide end - 28mm. Could the loose lens element be causing this? If so, this may just be a matter of tightening some screws.