"Popup" when visiting from a certain merchant/search engine

I have an interesting, to me, social experiment going on. There is a simple jquery pop up window that welcomes folks who come from a certain shopping engine. That popup simply says welcome and let us know if we are not the best place to purchase.

Since it was implemented the volume of purchasers from that engine has gone down. Now I don't know if it is coincidence or not but for over a month it seems to have a correlation.

It makes me wonder if it is true or just coincidence. And has me thinking what my reaction would be to a pop up that isn't blocked by my pop up blocker and requires an action by me to close it.

Mike