My theory is based on demand and popularity, mixed with little bit of game-playing on their part. They give everything a sales rank. I bet if a few people buy an item they are probably going to raise it. If there is something you want and its really discounted, you should probably get it rather than leaving it in your cart because it will probably just go up. That happens to me a lot, and its always on the best deals that are steeply discounted.
Im an amazon prime member now which is actually really good; and you get price protection if something goes lower than what you paid. The only drawback is the amazon prime orders seem to get processed right away so you cant cancel them. I like to be able to cancel the order if I get cold feet 20 minutes later.