Search ranking is with many websites directly proportionate to profit. With every update to the Google search engine algorithms, the overall search ranking and consequentially the traffic on the said website will go up or down. It is uncommon for it to go up and down in a relatively short time period.
In the last few days, SEO analysts and webmasters have reported their website ranking charts were all over the place. A possible reason is a kind of small update or a tweak to done by Google, though there has been no official announcement and there is some indication the tweaks are being deployed one after the other, resulting in very unstable search results. This erratic, unpredictable traffic has lead to frustration and annoyance with SEO communities.
Renown SEO forums such as WebmasterWorld bear witness to some reporting a loss of traffic between 70 and 90%, keyword dropping, movement in Finance sector … and rarely, but yes, some mentioned they actually have increased traffic.
The constructive thinking among those discussing leads to making the first-hand conclusion which indicates over-optimization in one or more SEO features may lead to Google penalizing such websites. Recent news that Google was demoting search ranking for websites that receive 80% of their traffic from search engines alone indicated that heavily optimized websites will be receiving penalties. Mobile first indexing might be the main culprit, but even that does not explain such wild traffic flux.
In the end, the most plausible explanation is that Google is testing an algorithm and they may need time to refine while the do more manual tweaking