My sites are relatively new about 3 years or so compared to many of the people here. Entertainment / leisure information niche.
Semi original content, I edit, compile, and create my editorial opinion and voice to be a information resource. Almost most of my information can be found at other resources, but I make sure that my information is the most complete and useful. (You could even say concise in some situations, as I take out the fluff). And my users do appreciate my work.
My first serious original site had 60k pv daily at the peak, I couldn't break through the "glass ceiling". I realized that my new content continuously pulled my traffic from my previous content. I couldn't figure out why, and I couldn't fix it no matter how hard I tried. At the moment this site stables at roughly 30k.
What I did is that I branched out and refocused my topics, ie still working on the original site with mini information, and made focused mini authority whereever is applicable. (When I have enough to say and if the need is enough) Which I think improved user metrics and CTR on the SERP itself.
My entire network rests about 100k pv at the moment. But I see very interesting movements on a small scale with how indexing is done, and how traffic is rewarded after a given time, especially for new domains.
Lastly, because entertainment niche is not "evergreen", I expect a 30%~50% average dropoff if I stop working on sites long enough. But I see that as a cycle of the business, I love what I do so I don't mind. I had a microniche that got popular with 24k pv's and now sit at roughly 2k 2 month later.
Back to the signals, what I am suggesting is that google is not doing a good job determining quality. Spam sites may actually have "better" user metrics in Google Eyes where they don't and should not in our eyes. In my space, I see a site that simply copy off 100% of forum posts, it ends up getting many links pointing to it, (and subsequent visitors from active forums). From forum posters who complain that their posts have been copied. And from other websites that simply don't know where to point to the original. The site has raised quite a bit and is at all time high. (Google see that people go to it and discuss on forums?)
I also see another competitor who has a forum page, with useless posts with exact duplicate wording, but different keyword titles. It tricks visitors into the site, click a bunch of stuff trying to find information that do not exist. I bet that site has like 5% bounce rate, it's designed to generate CTR's and further clicks while offering no observable value. This competitor is at all time high too and 5 times my size. This forum is only 1 year old.(Google see insanely low bounce?)
These two "bad" competitors beat out my SERP ranking quite often. I actually attribute some of my traffic loss to them. I do hope that Google gets them somehow in the future. Examples like them really kill the motivations.
I agree with the invisible wire, the metric may be something completely crazy and don't make sense. And I agree that I can be gone instantly. That's kinda why I am on here trying to avoid ever tripping it myself. I find learning from other people's experiences very helpful. It is a very stressful job after all, I understand what everyone goes through. I am one of the people that check ranking and earnings every couple of hours haha.
Back to your perspective, do you notice that your competitor is gaining the traffic while you are losing? Would you be able to deduce the difference between you two? For example, if your competitor is crappy and bad community, but they give out quick answers to the education questions. Users may stop looking, and Google may rank that as satisfactory for a certain query.
"Better" user metrics does not simply mean bounce rate, but a "imaginary" ranking score derived from google's black box. That's my conclusion from observing everyone's comments on the forum.
If it's not user metrics, it must be something else that trips. I think if anything, I believe any change has a reason behind it, whether or not it makes sense to us. That's how I make sense of why my competitors are beating me where they shouldn't.