Several times a year Google goes through some major tinkering with the algorithms it uses to rank Web sites. These major changes are different from the close to monthly changes you will see in positioning of sites. Around a year ago there was a very major change that dropped most sites by at least one notch. That is, from 6 to 5 --or whatever. The most recent change was almost as dramatic.
In the past few PageRank updates it has become quite apparent that Google is continuously raising the bar on PageRank. In their defense, with all of the reciprocal link building, link renting, etc. going on this was a natural reaction to the growing number high PageRank sites that attained those ranks simply by building or buying hundreds and thousands of links. -- Search Engine Journal