Must be a site issue rather than iOS though sury as doesn't happen on any other site.
It happens at lots of sites but, like me, STW may be the only site you browse where you experience it.
How and where you see it depends partly on the sites you use and partly on whether the 'shady' ads served by those sites target you or not. Obviously each site can serve up dozens of different ads and those ads are different for different users, so you'll only get the issue if a site is serving particular ads to you
The OS issue is that the advert (or rather code hidden in the advert) is able to take the decision on behalf of the user and exit the browser and open another app. Its not apple's or android's fault that ads are being exploited in this way but it should be possible for the user to turn off that functionality. Its an issue thats been around long enough to be addressed.