@jstacat can you please explain are you an Adsense Publisher, or an Adwords Customer and ecommerce site?
@glakes
Here are few quote that I am going to refer to
Gross product sales increased 147.65% in 2016 from 2015 sales. Much of this growth is the result of supplying product to OEMs and also to wholesalers.
I've mentioned it before, but I'm in a very small industry. Most here probably don't even know my industry exists, and if I were running a botnet I certainly would not waste resources on anyone in my industry. The number of competitors I have is so small I can count them on one hand and the market for what I sell is not widely consumed by the average business or consumer.
Most of my sales are the result of B2B transactions and many of these buyers are still using IE or whatever their network admins installed on their workstations. Chrome has a marketshare of 43.07% as reported by stat counter. Why aren't the other 57% of Google users buying or are you also suggesting that the web browser is also being spoofed in addition to the referrer?
First, congratulations, on what appears to have been a great year for you, 148% sales growth pretty good by any standards.
Maybe Google isn't your problem but a symptom of your success and the zombies could be the results of various factors. First, you are giving your customers many channels through which to buy from you. Such as, Direct online, direct offline (I assume), Amazon, OEM agreements and through wholesaler/retailers. With so many options, many users could be using Google to get to your site more for information than purchase, then choosing another channel to buy your product.
You mention that most purchasers use IE when buying from you. This in and of itself could explain the difference in conversion rate between Google and Bing. If I dive into my stats [in GA go to Acquisition, Source select Organic, then select Google then use secondary dimension select User-> Browser]
What I find is that:
Of user that came to my site using Google only 5.5% were using either IE or Edge. For Bing 66% of users used IE or Edge, and for Yahoo it was 15%. Now my niche is much different from yours, so your stats should look different, but I would guess that the difference may even be bigger.
So what this say to me, is that your buyers prefer Bing over Google due to the nature of the industry. That means that user coming to your site from Google are less likely to be buyers. It may still be people searching for info, and they will possibly return to work the next day and buy via Bing.
Why the change September 2015, that I can't say, but if I had to speculate maybe you had boost in rankings. More people are finding you but not necessarily buying at the time that they find you. But clearly your making the sale.