claaarky - 1:45 pm on Dec 16, 2012 (gmt 0)
A few people have touched on things that strike a chord with me.
Several sections of my ecommerce site don't convert well, however they rank really well and in fact generate the majority of my traffic. I've noticed that when traffic goes up its mostly increasing for these pages so conversion rate for the whole site appears worse, but it's just because more traffic is going to pages that don't convert well.
Then there's mobile. My site converts much worse for smartphones and at weekends in particular (and some periods during the week, such as lunchtimes and evenings) my traffic is mostly smartphone users. It's the same traffic but people are out and about instead of at their desktops.
If traffic rises at a weekend it's a double whammy for my conversion rate. More traffic going to areas that don't convert well AND more of that traffic is coming from smartphones which converts worse.
These low converting, high traffic pages have amazing click through rates from google, much better than the rest of the site, so I think, when more people are searching, I see traffic go up more on these pages because they are much better at enticing visitors in than my better converting pages.
This combination of factors is what I believe create the zombie effect, for my site at least.
I also think this is something sites demoted by Panda (like mine) are likely to notice more (possibly exclusively). I think the two issues are related.