That's what peeves me so much. I do not know.
I have not done anything special. I beat Urka in a fight, had Hiro fuck her butt, charisma checked her into the party next time we met, and I have done nothing but topping her now and then when camping, because both Trudy and Alma don't have topping scenes. Well, Trudy probably does, but you can't get to it without being topped first, which was against my goals.
Then one day, simply calling Urka over automatically initiates that dialogue and a scene of Hiro getting railed doggystyle in his tent.
At some point, I unlocked the Slut "perk", even though it's nowhere on the perk tree. Maybe if you have that and haven't been fucked by Urka in a while this happens. The wiki says nothing about such a scene, so I dunno. I'm just guessing.