"The globe effect, also known as rolling ball effect, is an optical illusion which can occur with optical instruments used visually, in particular binoculars or telescopes. If such an instrument is rectilinear, or free of rectilinear distortion, some observers get the impression of an image rolling on a convex surface when the instrument is panned."
https://en.wikipedia.org/wiki/Globe_effect
My layman's description: The field flattened image looks really nice stationary, but when you pan, it looks like the image is actually on a rolling cylinder, a convex image instead of the concave way our minds sort of interpret our sphere of vision. You might consciously perceive the effect, or if your brain has trouble processing this optical illusion, you notice it subconsciously and might get motion sickness.