human biology - Why does looking at bright light trigger sneezing in some people?

You are talking about the photic sneeze reflex.

The mechanisms are not entirely understood, but it affects 18-35% of the population.

According to Wikipedia (although this passage is not sourced):

The probable cause is a congenital
malfunction in nerve signals in the
trigeminal nerve nuclei. The fifth
cranial nerve, called the trigeminal
nerve, is apparently responsible for
sneezes. Research suggests that some
people have an association between
this nerve and the nerve that
transmits visual impulses to the
brain. Overstimulation of the optic
nerve triggers the trigeminal nerve,
and this causes the photic sneeze
reflex.

And, it could be advantageous:

On the other hand, some people with
the trait feel that it is
advantageous. In the event that nasal
discomfort occurs, but to an extent
that is insufficient to induce a
sneeze, intentionally seeking and
finding a light source facilitates the
sneezing process and is in turn a mode
of relief.

Also, anecdotally, as I too photosneeze, I can see an advantage: should the atmosphere become weaker in the future, and allow more light to enter than we are accustomed to, it would discourage looking directly at the sun.