That could be for a number of reasons
1. The PageRank of the websites linking to you went down.
2. Websites with a high PageRank stopped linking to you.
3. Google saw attempts of artificially increasing your PageRank.
Let's look at it from another angle. A high PageRank isn't something that one can directly achieve, it is a result of doing something right. If the website you're referring to has valuable content, users will link to it, and your PageRank will climb. If you're deliberately trying to increase your PageRank and focus on it too much, there's a good chance it will backfire.