Excellent question, and one most of my clients ask me. There could be a number of factors, price, short sale, trust sale, etc.
Just a thought... Do you have Realtor you can ask these types of questions to? If you are considering selling or buying, having the resource of a local Realtor can help you get a pulse on the market. If you are considering selling your home, don't be like the one you asked about. The longer a home is on the market, the more buyers thing something is wrong with the home.