What is this and why it opened on its own?

It should appear if you are on a public hotspot, like hotel, airport, cafe, university, you get the idea. Since you are actually seeing a server that is misconfigured this is probably the case. Or there is a temporary problem with your internet connection, and esp. with DNS servers. You can try waiting a bit and or restarting the home modem/router if you are at home.
So in either way with 99% probability what is broken is not on your end and you do not really need to do anything (except finding another hotspot if you are in a public place or wait for the provider to fix it if you are at home).

That said, that autodetection could be turned off somewhere in one config file, but i cannot remember where…oh wait i found it. But i recommend just waiting, before generally disabling it. It is still a useful thing when it works.