The way I understand it, the bright-light-sneeze thing is similar to phantom limb sensations. The part of your brain that controls pupils dilation is adjacent to the part that controls sneezes. Sometimes the pupil contraction signal "leaks" into the sneeze area, causing the sneeze. So bright sunny days, looking into lightbulbs, etc causes sneezes. I think there's a genetic component to it, like being able to roll your tongue.
