Well of course, you answered it yourself. Graphical glitches like these always occur when you alt-tab out of games and back in, making all the graphics reload and re-render.
Every time I alt-tab out and back in, clouds show up as blocks, the cockpit is completely distorted, etc, but I usually, I can just alt tab back out and back in to re-render everything and after a few tries it returns to normal, with all the glitches sorted out.
Just don't all tab out! ;)