Joined: 25 Feb 11
08 Mar 12 12:44 pm
It could be due to a number of reasons. It's possibly something to do with content. But it can also be because of the links you are linking back to, your article's relevance to your keywords, even having too much advertisements on your page can get your webpage/website de-indexed.
Most of the time, it is an entire website and not just one webpage that Google takes out of their search engine result pages. I suggest you go over the following article in the Google Webmasters Help page
on what to do if your site is not doing well in search.
Hope that helps. Have a good day!