This happens quite often. When I am browsing through safe websites suddenly a scary virus alert comes with a toll free number to call and it hijacks the browser. Sometimes I have to end the browser process from task manager to close the browser and get rid of that pop-up. I have not installed any malicious software on my computer.
This is probably malvertisement, i.e. malware or scareware (like in your case) delivered inside the sites advertisement. Or you have some malicious ad-injecting browser extension installed or some ad-injection software like Superfish. Hard to tell what exactly is the case.
In the first case it might get fixed by installing an ad-blocking software, at least unless the site in question currently tries to work around ad-blocking software. In the other cases check your system for browser extension or some strange software. Note that browser extension turned malicious in the past and that ad-injecting software often comes bundled with some free software.