Update June 2019 — I completely rewrote the content and the issue disappeared on those pages. I still don’t understand why this happens on certain pages. I simply rewrote everything and they stopped dropping.
A page dropped far beyond the top 100. For a long time I couldn’t figure out why. I reduced keyword density, but it didn’t help. Text analyzers showed no over-optimization anywhere…
Density for ‘best’ and ‘review’ was under 1%.
Checking rankings, I saw one page reach the top 50 for the key phrase «best product», then another, while the main page couldn’t reach the top 100 at all.
I removed the word ‘best’ almost everywhere, especially from headings. I left only 4 occurrences in the main text, although on the page itself there were occurrences in related posts + in the top menu some links had the key word ‘best’. I don’t know if occurrences in links count.
I submitted it for re-indexing in the new Google Search Console. The result 5 minutes after re-indexing: the page jumped straight into the top 10.
The most interesting part is that I had encountered a filter for a single word, «review», even earlier, but couldn’t figure out why. I noticed that for the query product review, my page simply wouldn’t rank in the top. It turned out to be over-optimization on that single word as well. By the way, that query is now in the top 10. So there you go. In the SEO community abroad, they say this is Panda and the main cause is keyword stuffing.
So reduce over-optimization if a page has dropped beyond the top 100 and refuses to bounce back.
Lower the density of individual keywords if, for instance, the product name itself with search volume of 14,800 is already in the top 100, while much weaker keywords containing ‘best’ fail to rank. It means there is keyword stuffing on the word ‘best’.
Analyzers I used (and mostly use):
https://copywritely.com/tools/
http://istio.com/rus/text/analyz/
1. You need to check word occurrences across the ENTIRE page, not just within the main text.
2. If a page drops beyond the top 100, keyword stuffing in the text is likely, and you need to remove a specific word almost everywhere in the text. In this case, BEST or REVIEW.
3. Just because a text analyzer doesn’t flag over-optimization doesn’t mean it isn’t there.