Shaddows - 3:32 pm on Sep 11, 2012 (gmt 0)
As I've stated in the past, the more you dig into the data, the less you see. It's really frustrating.
Our site is extremely top-heavy when it comes to traffic numbers- a small percentage of pages get the vast, vast majority of traffic.
In fictional numbers, say the lower-performing 80% of pages got 10-20 unique visitors per day, even in a bout of Zombie Traffic, they would still be getting 10-20 per day. But instead of some being up and some being down, the tendency would be towards the top end. Headline traffic would be up 10-20%, but any given page would be well within normal parameters.
I get the impression we're not typical of Zombie victims, so I'm not sure how that fits into the overall picture.
I've decided that analysis is not helping, so I've stopped worrying. It hasn't affected our bottom line, and it has taken up a lot of my number-crunching time over the months and years.
Currently, I'm thinking it is fairly benign. Google switches off a bunch of filters, or the Intent engine is turned off, or personalisation is dialed down. It always happens when the SEO chatter is about ecom disruption, so I'm fairly sure it's related to algo module releases.
We're not particularly Geo-targetted so we don't notice that filter like many do. However, we do rank very well for a mix of info and ecom terms- I can only imagine an Intent-Type that we haven't thought of has our pages supressed from their SERPs, but suddenly we get shown to them. They're not impressed and leave.
Most likely, the people who see quality traffic disappear are highly targetted in some way. Google could very well be hiding (normally) a bunch of irrelevant sites from your target customers. When the filters get switched off, all the irrelevant stuff starts competing again, losing you the good traffic. Meanwhile, searches you could rank for but don't (because G knows your not a good match for that particular person on that particular occasion) start returning your site, resulting in untargetted traffic. Geo-locatation would be a prime candidate for that.
In any case, unless I have a eureka moment, or someone has a truly inspired insight, I'm giving up on tracing this phenomenon.
Good luck to those materially affected by this issue. I hope you figure it out.