I'm not tech specialist but here are my thoughts,
-could be that your ISP, router, DNS requires a login based off of the url/server name (yeah really simple I know),
-that's really random, do you have any other tabs, windows open that are accessing the internet? it could be that that site is asking for authentication.
-you've been served...with a virus for PC... lol. seriously thought you could have some virus, malware, spyware, or some kind of variety if you've accessed sites that seem illegitimate. Maybe installl AVG Free, or Malwarebytes to scan for both.
-nothing else I can think of, unless like previous poster said it is a local connection of some sort. Is there anyone else connected to your network? Also how secure is your network? Could be an issue there where someone else (you willingly let them or not) is using your network and has access to your computer, if you haven't enabled the firewall and other network security settings and is again running some script.
Again bare with me. I could be completely wrong. Just wanted to throw out some ideas that I had out there. Maybe it'll spark someone else's brain for something of this nature.
Good luck. Hope you figure it out. People here will know if anything.
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