Big changes in the shape of the shadows can sometimes happen if the shadow is cast from some distant piece of terrain. When the shadow-casting terrain is far from the camera the subdivision levels are reduced (the shadow-casting polygons are larger), and when that shadow is cast onto something in the foreground the polygons of the distant terrain are effectively magnified. Level of detail changes can therefore become very noticeable.
If that is what's happening here, you may find that this region is in partial shadow if you enable soft shadows. There will probably still be differences between levels of detail with soft shadows enabled, but they will just be differences in how much light reaches the foreground rather than a sudden on-off switch.
Matt
