I agree with you folks, mobile is NOT the ONLY cause for Zombie traffic, but I think it's a big part.
I do agree that even the PC traffic has been of poor (non converting) quality all this year (Since April).
My thought is that Google is doing something similar to what grocery stores do from time to time...they rearrange the whole store so you CAN'T find anything...that way, in your search, you discover OTHER things you weren't looking for. Ridiculous, but it leads people through many more loopholes and ads than if they find what the need right away. Did I mention Ads?
Just this morning, my mobile traffic is at 36% - wow! 2 or 3 years ago when sales were booming and high speed mobile access not readily available, that might have been 2 or 3 %. At the end of a typical day, it's been around 23%. That's a considerable chunk of traffic.
I'm not so paranoid to think that Google is manipulating anything based on GA usage, but setting aside the mobile issue, there still seems to be a randomness to on off periods. I wish I could track those better, but since the 4th of September, I haven't had any ON periods.