This is usually due to an infected advertisement. Adverts don't come from the main site usually, they are provided by a third party server and security on these seems... neglected to say the least. That's the reason why I run an ad blocking add-on. Sure, most sites get revenues from third party ads, but too many advertisers lack security checks or took liberties to auto-start video ads or bigger than allowed ads. I had enough and blocked them all years ago.
1999 Craftsman 11.0/31 (C950-52930-0)