I'm not going to doubt or support any theory, they're all good but none hold any water yet. The serps just keep flipping as designed, but I'll share a little fact...
Today at about noon, out of desperation, I submitted a re-inclusion request. I figured now that I'm down > 50% that's a pretty good sign I'm being penalized for something and maybe, if I'm lucky, they'll reply and explain it to me (lol).
Well, not more than one hour later, I get a call from the Google Adwords team offering to get me a #1 spot in Google. Coincidence?
Remember the precept of Occam's Razor? The simplest explanation is likely the correct one? I see money as the most logical motivation for any serp position policy at the Big Box search engines (or any other venture on this planet), so that's my choice. It's currently the quickest and most reliable (not to mention expensive) way to improve your traffic. Oh, and if you think they're out just to improve "user experience", I'll get the smelling salts to you.
I think we've had enough correlation between members in the US and UK to know that "something is up" and it's some type of throttling for sites within a specific "goldilocks zone". We really haven't compared traffic volume on the affected sites, but I'd take a stab that the zombie zone is between 500 to 3000 uniques per day. Below that, it doesn't matter, everything is practically dead (barely trips the 2% rule), above that, smaller fluctuations don't mean as much. The heavy hitters are getting around 30k uniques per day and small fluctuations affect them like a mosquito hitting a semi truck windshield.
The rest of you who don't see zombie or throttled traffic periods, be thankful.
Over longer periods you may be experiencing content dilution, mobile infiltration and this thing called "diversity" that dilutes your positions further. It's all part of the continued monetization of the serps.
About 5 years ago, I took a motorcycle ride in the middle of the week, business was booming and on autopilot. I stopped at a bar in northern Wisconsin and it was empty except for some young punk bartender (who I found fixed PC's on his off time). He asked me what I do and I gave him the shirt version. We started discussing Google organic and paid listings. He insisted you could NOT get into G for free, you HAD to pay. I disagreed. It almost got ugly. Anyway, looking back it seems that kid was from the future! It's definitely moving in that direction.
BTW - regarding the number of contributors...I think many more are reading this and covering their hand. Others have just given up. If a tree falls and only one person sees it, does that mean it never fell?
I'm getting on the phone tomorrow to drum up some VC to kick start my Adwords campaign.