Simple physics. LCD screens typically use a polarization layer in part of their construction, so your polarized glasses are acting as a second polarization screen causing the image to obscure because the light from the screen is getting blocked. Take two pair of polarized glasses, align the lenses, then start turning them in opposite directions and voila! the visual field will completely obscure. Continue turning, and it will no longer be obscured.
You can see this same effect on LCD car radio dials, LCD wristwatches, LCD TVs, LCD calculator screens, etc.