Google update their index about once a month. Each time Google update othe database of web pages, the index invariably shifts: new sites are found, some sites are lost, and sites ranking may change.
Your rank naturally will be affected by changes in the ranking of other sites. You can be assured that no one at Google has hand adjusted the results to boost the ranking of a site. Google’s order of results is automatically determined by several factors, including our PageRank algorithm. Please check out our Technology Overview page for more information on how this works.
You may want to check and see if the number of other sites linking to your URL has changed. This is the single biggest factor in determining what sites are indexed by Google, as Google find most pages when the robots crawl the web and jump from page to page via hyperlinks.
Excerpt taken from Google Webmaster Info