The Wall Street Journal took a look at the reasons buyers default on their bids here in the US. What they found out was surprising and interesting:

At Dallas-based Heritage Auctioneers & Galleries Inc., people renege on their winning bids roughly 10 times a year, says President Greg Rohan. The most common reasons: someone dies between the time the bid was placed and the date of the auction; an unexpected tax lien is placed on a bidder’s bank account; divorce or medical emergency. Occasionally, bidders just get too caught up in the excitement and bid too high, Mr. Rohan says.

When Art Buyers Change Their Minds (WSJ)