These may just be pop unders, they open a window underneath the window you are viewing that needs to be closed separately. I have not seen one in a while but I have a pop up blocker in place, you’d have to check if your pop up blocker blocks pop unders.
There is a chance that you have become affected by adware, the will look at what searching for and bring up rogue sites. If you are using Windows Microsoft have a tool that will remove unwanted software, Google Microsoft® Windows® Malicious Software Removal Tool (KB890830), this will take you to the download site. The first time this is run it can take some time but it is probably worth it.
HTH