On one of my sites, I added a link to a new page from my homepage which included some of my keywords. It may have been a coincidence, but my site lost 6 positions. At this time I was also building links by article submissions. I also noticed a "comment" link had dropped off a nice PR page. Can too high of keyword density have caused this? I just changed the link last night to omit those keyword additions, and have noticed that some datacenters have boosted my SERP close to where it was. Do search engines change SERPs in reaction to backlinks and on page changes this quickly? My site is spidered pretty regularly, so is it possible to for SERPs to react in a day or less from on site changes?
I'd consult http://www.seomoz.org/article/search-ranking-factors for the relative effect of each SEO aspect on your SERP placement.
Theres to many other possibilities for why serps can change, sometimes you can convince yourself its down to a recent change when it may not be connected at all.