What are the main reasons a website or webpage might be de-indexed by search engines?

There are several potential reasons why a website or specific pages on a site could get de-indexed, meaning removed from a search engine's index and results. Some of the main causes include:

Quality Issues

Search engines aim to provide the best, most relevant results for any given query. So pages with thin, low-quality content, excessive errors, or an overall poor user experience run the risk of getting de-indexed over time.

Duplicate Content

Having large portions of duplicate or substantially similar content across your own pages or from other sites on the web can cause search engines to filter out and de-index those duplicated pages from their indexes.

Technical SEO Problems

Major technical SEO issues that prevent search bots from being able to effectively crawl and render pages can lead to de-indexing. Things like redirect loops, server errors, blocked resources, etc.

Webmaster Guideline Violations

If a search engine determines pages or an entire site violates their quality guidelines and policies around things like auto-generated content, cloaking, sneaky redirects, unnatural link practices, etc. they may apply a de-indexing penalty.

Manual Actions

Search engines can also manually de-index pages or full sites that they've caught trying to deceive their rankings through tactics considered spammy, fraudulent, etc.

No Inbound Links

Pages with no external inbound links pointing to them can eventually get dropped from search indexes over time if there are no routing paths for bots to discover the content.

Site Owner Request

Site owners can also choose to manually de-index specific pages or sections using the appropriate removal tools like Google Search Console.

So in summary, quality issues, technical problems, policy violations, and lack of inbound links can all potentially lead to de-indexing by search engines if not addressed properly.