Guys With Eyes in the Back of Their Heads

At last nights Twins game at the Target Field in Minneapolis, MN I saw quite a few spectacles.... and I felt that one of them was necessary to share-- guys who wear sunglasses upside down on the back of their heads! My question: WHY?

I NEVER understood why guys wear their sunglasses like this.  I mean, if it's cloudy outside can't you hook them on your shirt collar?  Can't you put them on top of your head?  Can't you put them in a sunglasses case?  Can't you just leave them in the car?  From my perspective, this looks like a device to measure brain waves.

Have you ever seen this look?