The website I am working on has a lot of news and much more content. We had the site since around 2010 and index was gradually growing up-to 350,000 pages. After 2016-08-15 we are losing indexed pages rapidly and now it is around 150,000 pages. (On 2016-10-25)Note : We did not change Sitemap, Robot.txt in any way and we did not encountered any index drops before. The dropping is stabilizing now.We identified following issues in our website.  We have meta keywords tags which can be marked as keyword stuffed tags.Most of our pages are not responsive and not mobile friendly.Google index status graph shows one removed page after the above date.We did not receive any manual action notifications or warning notifications from google. Could someone please clarify what could have caused this issue? Is it possible google decided to de-index pages based on above listed issues or on one of their algorithm update or a penalty? Update : I am not a SEO expert. I am a front end developer who just noticed this. 
Losing indexed page count rapidly since 2016 August 15
google search console;google index;keyword stuffing
Google generally removes pages from the index for one of three reasons:It can't crawl them properlyIt found that they duplicate something elseNobody searched for them and they don't have many external linksFor crawling, check Google Search Console and make sure that there are not unexpected crawl errors.   Check your server logs and see that Googlebot is fetching your pages with a 200 status.For duplicate content, search for a sentence from each page in quotes on Google.   You may find that there is a scraper site out there that has stolen your content.   Google occasionally indexes a scraper site instead of the original source of content.For lack of interest, there isn't much you can do.   I've observed that Google will often crawl and index a page for the first couple weeks before deciding that it isn't high enough quality to continue including in the search index.