This happens to me too, usually when I've been indoors the whole day and decide to step out. From what I've read, there is no scientific research to conclusively explain this phenomenon. According to a popular hypothesis, this light-induced (photic) sneeze reflex is caused by a confusion of nerve signals in pathways very near one another. Since sneezing is such a sudden and involuntary reflex, the cause is probably located in the nervous system, which is capable of transmitting signals very quickly. Researchers suspect that two important reflexes may play a key role in sun sneezing. The first is the pupillary light reflex. In this reflex, bright light entering the eyes sends signals along the optic nerve to the brain, which sends signals back to the eyes to constrict the pupils—a means of adjusting to differently lit environments. The second is the sneeze reflex, in which a cranial nerve called the trigeminal nerve detects a tickling in the nose and alerts the brain, which in turn stimulates the chest, nose, mouth and other muscles involved in sneezing. http://scienceline.org/2009/11/why-do-some-people-sneeze-when-they-look-at-the-sun/
Photic sneezes are not uncommon, so please don't be alarmed. It is not harmful to your child and does not lead to any colds or infections. If you baby is sneezing in episodes, this is expected and is nothing to worry about. In the simplest of explanations, the ACHOO (Autosomal-dominant Compelling Helio-Ophthalmic Outburst) syndrome is caused by a confusion of body signals. It's not uncommon, it's not a threat to your child so please don't worry. http://www.scientificamerican.com/article/looking-at-the-sun-can-trigger-a-sneeze/
It is called ACHOO syndrome where it affects 20% of the population. Like Santiago said, it's not uncommon and nothing dangerous so don't worry about it. https://en.wikipedia.org/wiki/Photic_sneeze_reflex