Google’s head of search spam, Matt Cutts posted a video explaining why it is not uncommon to see a new page rank very well in Google early on, and then, that ranking declines over time.
Matt explains that sometimes, Google has a hard time figuring out the original source of a new piece of content. But over time, i.e., days, weeks, months, Google is better able to figure out the most relevant result for a query due to indexing more signals over time. Thus, over time, the search results may settle down to a particular state; but early on, new pages may rank high and lose their rankings over time.
Certain queries that deserve freshness, that is where QDF – Query Deserves Freshness comes from. Other queries prefer more evergreen, more settled content, Matt explains.
Here is the video: