Im not sure what causes the circles. There are MANY windshields and automobile windows that cause similar effects. Window tint looks crazy when viewed with polarized glasses. I think it has to do with the UV protection built in to many windows. For fun look at other vehicle's side and rear windows with the glasses.
I think (searching brain for long forgotten physics classes) that essentially what you're seeing is due to minor warping and thickness variations in the plexiglass, or auto glass viewed from afar for that matter. Light generally bounces around at all kinds of crazy angles, and polarizers act somewhat like "venitian blinds" and make all the light hit your eyes at the same "orientation." That why if you put two polarized lenses back to back, at different orientations, no light gets through. I think the color show happens since light going through the glass/plexi gets scattered due to the imprefections. The polarizer filters some of the imperfect light, causing only limited portions of the spectrum to hit your eyes (remember "white" light is made up of all the colors in the rainbow ROYGBIV).
Sorry, it's been a looooooong time since i took a physics class. I do remember lots of cool demos and theory behind polarization.
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