If you look closely at other areas, it's clear that there's also a single point that everything is rotating around, albeit not a center point that lies on the intersection of grid lines.

I think much of the ambiguity is because of the frame rate. Squares in the outer portion moving something in the neighborhood of two units per frame according to the "global" interpretation, but at the same time if you're looking at one of the corners it's easier to see a separate rotation centered over there because the brain has an easier time working with an interpretation where each square is moving a little bit between frames than with one where each square is teleporting across a relatively large distance between every frame.