1. Lack of a common theme throughout the site. If a website is about cell phones, home improvement, banana desserts, and online education, it will require about 100 times as much authority to rank well in search engines. Adding a new section to your site that is completely unrelated to your site could cause a slide in rankings or could prevent you from getting ranked in the first place.

2. Use of doorway pages to create content. Doorway pages are pages that provide little or no value to anyone, but are created only to make a page rank for a certain keyword. A very common method of creating a doorway page is to use the words that are on another page, changing the keyword at the top. If I made one page for ‘search engine optimization‘, and another for ‘SEO’, I would need to create separate wording and content for the SEO page - otherwise it would be considered a doorway page. Each page in your site must have separate copy (wording).

3. Bad link structure. A person that finds the homepage of a site should easily be able to click to any page within the site. Ideally, the visitor should be able to find each page using at least a few different paths. This means that each page within your site should be linked to from at least several locations. This increases PageRank for your pages and makes each page look more important to search engines. Changing your link structure can be dangerous, especially if it decreases the amount of internal linking in your site.

4. Getting out-gunned by the competition. If your competitors are developing better content and are working harder on their link-building campaigns than you are, you will eventually lose your search engine rankings. Getting to the top requires a lot of effort and although staying there is much easier than getting there, it still requires effort.

5. Getting caught while using black-hat techniques. For a more thorough explanation of these techniques, check out this one from the archives: 10 Simple Ways To Get Banned By Google.

6. Modifications to search engine ranking methods. Google, Yahoo!, and MSN all update their ranking methods (algorithms) periodically. Increasing your authority (inbound links and site size) can help you to avoid being affected by changes in search algorithms.

7. Loss of links and PageRank. Websites change and get deleted all the time. If your site isn’t attracting new links, the loss of existing ones can be devastating.
There isn’t usually anything you can do about link-loss, except for continuing to attract new links.

8. Lack of link growth. Good websites will naturally attract links. If your website stops getting new links, that stagnation can cause you to lose your search engine rankings. To a search engine, the lack of new links means that people are viewing the site anymore.

9. Lack of updates. Websites are seen as living, breathing organisms by search engines. They want to see action in the form of updates. A website that never changes won’t be seen as fresh, and this can cause your rankings to drop. For best results, update at least once a week.