Each time Google update the database of web pages (about once a month), the index shifts. If your site was dropped from Google and you have not made major changes to it in the last month, Google will likely pick it up again in the next index. It’s possible your site was simply inaccessible when the robots tried to crawl it.
You may want to check and see if the number of other sites linking to your URL has decreased. This is the single biggest factor in determining what sites are indexed by Google, as we find most pages when our robots crawl the web and jump from page to page via hyperlinks. To find out who links to your site, use Google’s link: tool.
It’s also possible your rank decreased because other sites were found and assigned a higher rank. 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 the PageRank algorithm. Please check out Google’s “Technology Overview page for more information on how this works.
Excerpt taken from Google Webmaster Info