There's a nice paper in the American Economic Review at
http://www.economics.harvard.edu/~aroth/papers/eBay.veryshortaer.pdf
Despite the paper's title, which, by scientific law, must be long and dry, the paper does go into the reasons that people snipe, based on surveys.
Some interesting factoids:
10% of snipers felt that they would be giving away valuable information about the value of an item by bidding early.
Under 10% of snipers apparently sniped because they didn't understand how the proxy system worked. These were mainly eBayers who had never bid before.