I have seen a similar problem with airline tickets before. After talking to a rep at AA, they had the following explaination.

When you get a price quote, the system reserves you a room. When you leave the page (like clicking the back button) its supposed to then release that room. Supposedly, some of the browsers don't do this release properly. So then as you keep checking the price you start reserving more and more rooms. At this point the pricing systems detect a run on capacity and raises the price accordingly. It doesn't seem like this would affect hotels as drastically as airplanes, but its just a though.