When you apply a drop shadow to a graphic, and the shadow drops to the bottom-right, and that's fine... but when you rotate the graphic, the drop shadow rotates along with it. So a 180° rotation would put the shadow at the top-left of the object. This causes problems when you animate the rotation of a clip that has a Drop Shadow effect applied.

Drop shadows are meant to depict a shadow an object casts when light hits it. If you rotate an object in real-life, you're not rotating the actual light source. The shadow would still extend in the same direction, no matter how the object itself was rotated.

The Drop Shadow effect needs a toggle box to ignore the rotation of the clip that it's applied to.