Scientists are baffled by it, but I'll hazard a guess. I've always thought that the abrupt change from dark to intense light causes the optic nerve, or its buddies, to vibrate. This causes tingles somewhere in my head that radiate to whatever nerve cluster causes a sneeze. At least I think that might be it. But it's only good for two or three good hearty (and I mean honking) sneezes. Maybe four on a good day. But after that, I'm all done. And I can stifle them with sunglasses, or a very conscious effort. But when you've got a talent like this? I let 'em fly. But it's funny. Most times, rather than "achoo," my sneezes almost always come out, "ah-duh-duh-duh-duh-chaaahhhhhhh!" Loud, and stuttery, and really hard to squelch. My step dad's are always like, "Huh-DAH-chuh!" Maybe we all have our own special sneezes?
I'm still dubious about the "up to 35%" of people being affected by photic sneezing though. It always seems to be just me. The best part is that it's called "ACHOO." That's Autosomal Dominant Compelling Helioophthalmic Outburst (yeah, they kinda cheated).
Read more about Photic Sneezing at: Wikipedia
