IIRC, each dungeon only has so many instance servers available for it. That's why you used to get WP/AK queues where you'd have a full party, but be waiting on an available instance for 30+ minutes. I suspect that, if a roulette party is formed entirely of rouletters, it throws you into whatever dungeon has the most instance servers available.

And that's gonna be Sastasha, 'cause ain't nobody queuin' for that.