One of the most common occurrences with new web pages is that they reach a very high ranking on Google and in the first couple weeks of publishing, then experiences a slow ranking drop.
As this is a continuing trend for newly developed pages that have quality content, Matt Cutts has an answer to why this happens.
When a new page is published with quality content, the freshness of the content raises its ranking very rapidly. It takes Google a little while to investigate where the content came from. Time is taken to determine if the content is original, or just a way of retelling a story. After a few days or weeks, Google makes a decision which can affect the original ranking.
Matt Cutts highlights that Google is just taking its best guess based on the quality content, the keywords, and the freshness of the content. As time passes, more information becomes available. Everything then gets incorporated in the Page Rank algorithm. Ranks usually change over time as the content of the pages change, links change, and certain information becomes more popular than the other. The algorithm adapts to all these changes, and assess the web pages in order to provide the best results.
