Browser Hijack

I have a Nexus 6P running stock Android 6.0.1 and am getting fed up with what I can best describe as random browser Hijack. I can be using Chrome or clicking through an article on Facebook, it is usually something that is in a slideshow format, when all of a sudden I get either an alert that my Android device is infected by a virus, or "congratulations (ISP) customer, you won..." Where the ISP is whoever my data is going over.

My phone is not rooted, I'm pretty careful about the apps that I install and checking what access they want. Why do I keep getting these messages? The most annoying part is that they completely replace what I was looking at so if it was a multi page article, I have to close and reopen everything to find my way back, to maybe have it happen again. I see this most of the time on my AT&T wireless or Frontier DSL. My sister also sees this with her iPhone. Thanks!