A sneeze is usually triggered by an irritation in the nose, which is sensed by the trigeminal nerve, a cranial nerve responsible for facial sensation and motor control. This nerve is in close proximity to the optic nerve, which senses, for example, a sudden flood of light entering the retina. As the optic nerve fires to signal the brain to constrict the pupils, the theory goes, some of the electrical signal is sensed by the trigeminal nerve and mistaken by the brain as an irritant in the nose. Hence, a sneeze.
I believe this. The effect of sunlight does feel like an irritation in my nose. According to the article, 10 to 35 percent of the population are photic sneezers. It is said to be an autosomal-dominant trait that can be inherited from either parent.
The article hints that it's an embarrassing trait. Perhaps. It's also terrifying at times. Imagine being in fast, heavy traffic and having a violent, seven-sneeze fit. :-(
Autosomal-dominant Compelling Helio-Ophthalmic Outburst syndrome, or, ACHOO!