One of our clients called about a drop in Google PageRank for their website. The question was "What is going on and what do I do about it?" The short answer is that Google has made an adjustment in their algorithm for creating PageRank and that at this point it doesn't look like it affects search results. If your PageRank has changed and you don't think it should have, wait a couple of weeks and if it hasn't changed back you can ask Google to correct it.
Google did this because they have not been happy with websites that sell links back to your page. They don't see this as a natural process of increasing page reputation. It is short-cutting the process, so this is their policing effort to stop the more obvious paid links. Remember that Google is in the ad selling business as well.
Many of the link brokers are known, so their sites and the links they have are easy to spot and take action on. It is also a simple matter to spot the link ads just from the placement on a web page. Usually they are listed down the right side and/or across the bottom.
The long answer has two parts to it. The quick part is to know that the publicly visible Google PageRank is only a small part of how a page ranks in search results. Look at it as a general comparison between websites, not as the authority. The long answer is in time. Build the reputation of your website over time by investing a few hours on a regular basis to gradually build a first class site. Make sure that the keywords you want in your website are what the search engines see. Then work on quality links back to your website, for this is the authority Google does look for in your website.
There are other theories about what happened and I think this is the best explanation I've see. If I see other information I'll pass it on.