If I'm not mistaken, it was because the optical nerves are sorta connected to the nerves around your nose that make you want to sneeze. Bright sunlight sorta overhauls the optical nerves, they send a buttload of signals to the brain, telling the brain there's a lot of light, but, because these nerves are connected to those around your nose, some of these signals travel through those nerves in your nose, accidentally telling the brain that something's in your nose, making you sneeze.
Either that or I got nothing...