Welcome to the Webroot Community,
The pop-up ads are usually caused by an ad-supported extension for Internet Explorer, Firefox and Chrome, which is distributed through various monetization platforms during installation. The malicious browser extensions is typically added when you install another free software (video recording/streaming, download-managers or PDF creators) that had bundled into their installation this adware program.
What you are seeing and describing sounds like it may be what we on the Community refer to as a PUA. (Potentially Unwanted Application) These are very annoying at best in that they cause pop-us, redirect your browser home page, and other behavior that may slow down the computer and direct ads your way, but they are not actually doing anything bad like damaging files or stealing information. Often they are installed intentionally by you the user as browser add-ons for various tasks such as quick search tools.. but they also come with the result of added annoying pop-ups and ads. Other times they 'piggy back' with other software that you installed, or try to 'sneak' onto your system entirely.
WSA does detect and remove many PUA's, and more are being added, but WSA does not detect all of them. A simple browser add-on with PUA behavior that is easy to identify and easy to remove is not likely to be detected and removed by WSA. Those that are intentionally difficult to locate and remove are. Please see THIS LINK for more information regarding Webroot's stance on these annoying programs.
For those that are not detected by WSA, please see this KB Article. It has some easy to follow directions on locating and removing PUA's. You may also want to submit a Trouble Ticket, especially if you cannot remove it easily from the directions in the KB Article.
For those that ARE detected by WSA, but cannot be removed automatically, you can submit a Trouble Ticket. Webroot Support will help you get these annoying 'crapware' off your computer at no extra charge, and the additional examples may help to better automatic removal of that particular PUA for all users in the future.
Hope this helps!
Microsoft® Windows Insider MVP - Windows Security