rholcom2 is right. The light coming through a window becomes polarized. When it goes through your polarizing sunglasses the light is polarized again. The stripes you are seeing are interference patterns caused partly by differences in the coating and partly by stresses inherent in the window glass. To see an interesting effect, take your sunglasses off, look through them while holding them away from your face. Rotate the sunglasses back and forth and see the stripes change.