My site has been indexed as regular but I just found one page has been de-indexed. Those page has unique content but disappeared from Google listing. What happened?
I have experienced this a lot with some of the pages on my larger site, it generally happened to pages inside the forum and pages with a bad links:text ratio, I increased the amount of copy and KW density on the page and a few came back. Forum pages never did though. I think there is just a trigger for relevant content that you loose out on if you don't have enough relevant copy on a page.
This is due to saturation issue. Your that page got low links as compare to other pages which are becoming more stronger than that page, which results in gradually loss of importance of that page and finally in de-indexing from search engines. you can get it index again by gaining some links for that page.