During somewhere in April google did a major update and announced that they would always like to rank sites which have authority or have much more content. Now they seems to be getting to those webmasters who have network of micro niche sites and are earning well maybe in the range of $100+, of course, those earning low don't matter much to them. Some articles and reads on this topic can be found here: How to Make Money Online with SEO Make Money Online With Adsense Properly – Google Banning Many Websites Day 26: Uh-Oh, I’ve Been De-Indexed! If you are into micro niche adsense business, then it is time to do certain things listed below: 1. Add more content and pages into your site 2. Remove adsense ads from some of the pages, do not place ads on all the pages keep 5% of pages off ads 3. If possible, avoid adsense ads on homepage 4. Wherever possible do not keep all the ads on all the pages above fold, keep 5% of pages with ads below fold. Though i personally don't believe in micro niche sites as always hated MFA sites or micro niche sites as personally they don't add any value, i have been into building huge sites with heavy content (more than 500 pages in few sites) and did the SEO and waiting game. It paid well, though it took more than 2 years, but was worth the wait. Now i just rinse and repeat. If you have micro niche sites, it is time to convert them into full fledged websites, might need more investment and effort from your side, but that will be worth it.
Google is always looking to get rid of MFA sites. That's it. It doesn't matter how big or small. If you make MFA's you are not creating value for their advertisers and it will catch up to you.