Thinking about it some more it might have something to do with the wavelengths of UV rays. 99% of the UV radiation that reaches the earths surface is UVA which has a wavelength of 400 nm - 320 nm. My guess is whatever wavelength is not generally absorbed by the surface of the earth and goes back to the sky may not be strong enough or the right wavelength to break through the clouds. So it bounces back to the earth where it eventually enters our eyes and kills us. That's just my guess.