Exactly the same thing has happened to me. In about a month after starting a new site, it unexpectedly reached the first page in MSN search for quite a competitive word. I decided to improve the results by increasing the density of the key word the site ranked for. So I just added the words here and there on the page bringing the keyword wensity to 6%. No other changes were made. Despite this effort, my rank for this word suddenly dropped to some 50-s place within a day after the change. A week after that it dropped even much lower, so these days I do not even get any traffic with the word which used to be my main source of bringing visitors. I can’t explain it, but it is definitely not a coincidence. The change I made somehow affected my ranking.