It's based on the lens angle of view and panning speed. In the Cinematographers handbook, there is a table of Critical Panning speeds that must be avoided when shooting at 24, 25, or 30 fps. It even causes problems with horizontal camera movement or pans. This is the same stroboscopic effect that makes wagon wheels turn the wrong way and helicopter blades appear to turn slowly or change speeds. If the text is small, you need to have it move an even number of pixels per frame. It's just like trying to animate a title roll. The softer lines caused by scale tend to hide the problem. This looks like a judder caused by the stroboscopic effect of the thin lines moving in an uneven multiple of the number of pixels per second.
