Of course, google should be the source on the google page rank algoritm, but their explanation is very simple. I then found the original article by Sergey and Brin on the PageRank algorithm, which of course is a bit academic. I then read Ian Rogers' "Page Rank Explained", which was quite informative but still gave me no answer on what went wrong. The former two links were a bit technical (which can be good), whereas the article Google PageRank: What Do We Know About It by Smahing Magazine provides some good overall conclusions. Of course, also wikipedia has an explanation on PageRank.
Unfortunately, I am still left with an open question on want happened. I know I have not changed anything or tried to do any SEO. What I suspect might have happended is something in the lines of:
- my (relatively small) pagerank of 2, might have been based on a small number of high ranked incoming links
- some of these sites have changed their pages linking to me in ways, that gives less ranking to me from them (for instance, by having a lot more links on the page than the one to me)
- google has been running one of its updates of all the page ranks, which is suggested to happen periodically