Reasons for sudden drop in ranking for certain keywords of your website
All the reasons below are based on simple logic on how Google and other major Search Engines work. Here I don't think about the possible Google penalty that your website is appearing in good positions by different keywords.
1. Possible Google dance which reduced your keyword rank
May be it is the Google dance which caused sudden drop of your website ranking in certain keywords. If it is the real reason, you may recover your keyword rank with in some weeks.
2. Possible Search Algorithm update
Search Engines are always in their way to provide better results to its users. So they update their Search algorithm every time, some algorithm update may badly affect your keyword ranking.
3. Competitor is doing better than you
If competitor succeed in gaining more trust in Google for those keywords, they will appear above your website. It clearly shows the basic rule survival of fittest.
4. Lost backlinks with those keywords as anchor text
If you need to rank better in Google search result, you should have high quality backlinks with proper keywords as anchor text. If you lost some quality backlinks with those keywords as anchor text , it will affect your search ranking position.
5. Google manually removed the juice flowing from spammy links
If you are in a way to create backlinks in artificial way and Google find it, they may remove the value of those backlinks and as a result your keyword ranking will decrease. This situation may look like Google penalty for your website but actually your domain is not getting any penalty. Here Google only removed links which look like spamming and due to this the backlink count of your website reduces.
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