...after you defeat Tchort and SGS are hostile towards you.
Basically SGS went hostile towards my character, and I actually don't know why exactly, but it's most probably because Gorsky went hostile towards him in Core City at the Black Crawler's base. While I was trying to exit the area and fight the mutants, Gorsky and the Zone Rats went hostile on me.
Not a biggie as is, but it seems North Underrail doesn't appear as a selectable option even after Six tells us about it.