My iPhone used to do it constantly and only on this site. If you can, disable JavaScript on the Safari browser. It got rid of all the shat popping up for me.
I used to get them until my wife spotted on one of them in the top right hand corner Ad choices she clicked on that and you were given different options on wether you wanted to stop all pop ups or just some of them etc, she just clicked on no ads and I've had none since, that was a couple of months ago, I'm in the UK but I assume it applies in the States as well.