Have you noticed this on an active listing? If so, there could be various reasons. If it's a short sale it could be that the bank did not like the listing price and had them change it once they received an appraisal on it. If it's a normal sale, sometimes a seller feels that a higher price may get more attention, which is rarely the case.
If, however, you are talking about a price that starts at one point then goes down over time, that is a normal product of having the home on the market and not getting enough activity or getting feedback that it is too high and the seller reduces it to keep up with the market demands.
Each situation is different so I hope that is helpful.