Banditoseo's theory seems somewhat far fetched imo, but I get where it's coming from. Google is slowly pushing down organic results with ads and destabalizing the SERPs could be the next step to get businesses to invest in AdWords.
In the mean time I'll keep tweaking my methods. One of the things I'm doing now is diluting my SEO. Mixing up my external and internal anchors more and more, less on-page (longer permalinks, lower KW%), more outbound links etc. This way I'm hoping to stabilize the rankings.
Admittedly far-fetched. I'm just trying to reason my way through what I'm seeing in my niche. I could rattle off a dozen white-hat sites that have SERP trackers that look just like yours.
Here's one purely white-hat site, DA > 40:
No penalty. No notices from Google. Didn't do anything unusual for it. Nonetheless, huge variation in SERPs and overall traffic. It apparently took a hit from the unconfirmed algo update in May/June (Moz detected one, Google hasn't confirmed), but now is recovering, again for no clear reason Only analysis I saw of that update was here
https://www.searchenginejournal.com/data-suggests-google-algorithm-update-occurred-june-2016/167088/ and it really wouldn't apply to this site, which has thorough, original content.
I look at the site's competitors and I'm downright baffled. There are some competitor sites that have good reasons for beating this one, but those aren't the ones high in the SERPs. Most of what's high in the SERPs is either really new (and then it vanishes in less than a month) or is loaded with spammy backlinks.
Maybe indexing issues? Don't know about yours, but that could be an issue on this site, given the volume of content. As in, even the slightest problem with indexing will get you hit?
Either way, something is up, and I haven't seen any good explanations, nor can I infer many from examining competitor sites.