you analysis your site again and check out where fult your working and read some google guide lines........than after you find out why decreasing your site page rank.
update your site, put up more content and submit a new sitemap/ping google with it. I try to automate my updates to once a day or every other day for some sites to keep them fresh and keep google bot coming back. Also if you dont have robots.txt setup do that. What type of site are you running? i know its easy with a wordpress blog you can use google-sitemap-generator. That's what I use on my blog works great. If you don't use wordpress I'm sure there's something out there for you or build it manually yourself.
While it's technically true it could be reported (say, via Adsense report site) I think he was just trying to boost his post count and nothing more. Pagerank generally seems to be decreasing accross the board, I think Google has updated it's algorithm and theres possibily less Pagerank to go around. A whole market has evolved based on the value of Pagerank and I'm pretty sure they never intended it to be that way.
Maybe google wants to lower all sites.. so the quality to get PR 4 & 5 is much higher than it is now.. getting PR4 isn't SO hard..it takes some work, but isn't that difficult..PR5 bit more difficult, but that's why I think it lowers... Lowers here too so... Greetz
Your pagerank can reduce 'naturally' if you don't get new backlinks and stop updating the website. In some cases it might also be a penalty.
The primary reason for PR decrease is a decrease in BackLink PR value. Many of the links we get are there for 6 months but after a year they vanish. Or if you have links from blogs, forums, articles, etc., the hosting site "archives" the pages with the link and therefore you lose the link all together. The second reason is that pages within your site have been removed due to deleting, broken links, etc.
Now google is always analyzing the sit and frequently crawling, also updating twice in a month so if u don't do proper both on and off page optimization like this depressing will happen
twice a month? my keyword that is very low volume of searches changes twice a day it seems like... I bounce from #7 in the rankings to number 12-13 it seems like every other day, the google dance is annoying!
Maybe you lost some important backlinks. PageRank will decrease if you got penalty for selling links as well.
You can use the google webmater tool to know the overview of you website, maybe you can get some answers from there thanks!