This is not a strange thing when your website lose page rank or see drop in page rank. Google is always transparent and made everything clear why it ranks and website better. If you lost a page rank, then there are some common and valid reasons for this.
These are some common reasons why page rank of a website drops:
1. Selling Links
This is the most common reason for drop in page rank. If website has caught in selling links, it will surely see drop in page rank in next update. It breaks quality guideline and Google hates selling link. If your website’s PR went down due to this reason, you should remove all paid links on your website and then submit a reconsideration request from Google Web masters tool telling them that you have removed all paid links.
2.Too Many Paid Links Coming to Your Website
Google clearly mention that it hates when a person purchase links to increase website’s page rank. Google says that you can have some but not too many. If you are advertising on some other blogs, you can use nofollow attribute in those links. If a link passes page rank juice, Google cleverly monitor whether it is a paid link or not. If it found paid links towards your websites, you may see drop in your website’s page rank in next update.
3.High PR BackLink is no More
We all know that PageRank of a website depends on the backlinks of the website. If a website has some high PR backlinks, it will also get high PR in next update. If your website has seen drop in page rank there may be a possibility that the high PR link coming to your website is no more or removed my that website. If no backlinks, then why PR. You can use tool like Ahrefs to monitor newly added or removed backlinks and you can work on building more quality links, once you loses a high PR backlink.
4.Linking to Bad Websites
If your blog has links to some bad websites (porn and gambling), you will surely see drop in page rank of your website. Google strictly hit those website which passes page rank juice to these kind of websites. So try to avoid it.
5. Poor and Thin content
If your website is producing thin and poor content, it may also hurt page rank. I have seen that so many blogs gets PR 2-3 just because of their content quality only with few back links. If you start producing poor content, then there may be a possibility that you will see drop in page rank of your website.