​I have found some fantastic articles (see links below) that has been very helpful for me. Very depth insight.
www(dot)searchenginejournal(dot)com/17-ways-to-get-de-indexed/53537/
jacksonlo(dot)com/google-deindex-reindex/
www(dot)sitepoint(dot)com/seo-disasters-happens-google-de-indexes-site/
moz(dot)com/blog/logic-meet-google-crawling-to-deindex
1. NoIndex Errors
Solution: For blogs, check your Privacy settings (In WordPress Dashboard -> Reading Settings). Make sure you have clicked to allow Google to index the site or the page. Check that you have not used the ‘noindex' parameter either in your robots.txt files in the root directory, in the individual web file directories or in your Meta data (in the Head section of your page HTML).
2. Domain Name Expired
You forgot to make your domain name payment. Your site naturally disappears and may be completely deindexed if you fail to make your payment quickly enough.
Solution: Check that your domain name has not expired by simply accessing your site. Keep an eye on its expiry date, and renew it before then. Even better is to login to your domain name provider and click on ‘Auto renew' or similar. This it will automatically renew your domain payment when the time comes - if you have money in your account! You may have to wait to appear again in the rankings, but your site should be indexed again fairly rapidly.
3. Your Server Crashes
Most servers are pretty reliable, but it might be that your web host's servers have crashed and have not been restored before Googlebot's next visit. It is feasible that this can lead to your domain being delisted or even deindexed altogether.
Solution: If you find you have been deindexed, check with your web host. Have there been any crashes recently. If not, your site should reindexed at the next visit and appear in the listings during the next ranking change. It may take time to recover your original ranking.
4. Broken Links
Many people have lots of outgoing links on their site that they have never checked for a long time. These might be adverts or links to sites with similar content as your own. Through time, many such sites can disappear. Too many of these links that lead nowhere can result in Google punishing your entire domain by delisting it.
Solution: Use Google Webmaster Tools or a free link checker to check your links. You will find a good free link checker here:
http://www.drlinkcheck.com which lists all links on your site and highlights those that no longer work. Run this monthly to make sure your links are still live.
5. Excessive Outgoing Links Created too Quickly
If you generate too many links from your site out to other web pages in a short period of time, Google may deindex it. Google might regard this as evidence of you being involved in link-sharing or buying links. The search engines frown on such activity. You might also have been hacked by somebody who has hidden their links on your site.
Solution: Build your outgoing links slowly, in the same way as incoming backlinks should be added. Never use linking software or purchase links! Google will hit you hard if it detects this. What Google is looking for are natural backlinks created by people impressed by your content, and wanting to let their own visitors see it. This actually gives them more ranking points, because it is a positive service to their visitors. Artificial links can be deadly to you.