This is a frustratingly common problem, the answer is crappy advertise javascript. I have reported this problem numerous times to various sites such as anandtech, etc. Most commonly for me it only happens on iPhone and redirects to some kind of you won an iPhone ad.

I had some luck reporting this to publishers but they feel basically powerless, they have to blacklist these advertisers manually and they always come up with new tricks to get around it.

And as much as I would say, dump these ad brokers, I guess the reality is that it's hard to make money in online news at the moment, and these ad networks are the best they have.