As far as I know its some of the advertising banners are compromised... usually happens around the holidays when they are not as thoroughly checked.
"Free Iphone" alarms going off etc etc... its worse on mobile browsers... normally have to do a quick double tap on the back button to get back to the thread.
It's not just on here too happens to some fairly major news sites from time to time.
Having said that if its happening on all sites that you are visiting its your PC.. try a adware cleaner like adaware or malwarebytes... the basic free version is usuallyt sufficient and you can run it and remove afterwards if you wish.