This is due to static electricity. When you wipe a CD surface (plastic) with a cloth, it rubs extra off extra electrons onto the CD's surface, thus imparting a negative static charge to the CD surface. Airborne dust and lint that come close to the CD get affected by this charge. The electrons in the dust are pushed away from the side facing the CD, thus leaving a partial positive charge on the side of the dust facing the CD. Opposites attract, so the positively charged side of the dust is attracted to the negatively charged CD surface. Because the dust and lint are so small and light, they move toward the CD and stick to the surface. This means that you use the cloth to again wipe the CD clean, thus starting the cycle all over again.