Would be sad if those triggers were broken though.
There is one S-Rank mob where I'd say that it's pretty obvious what you need to do:

Name: Nunyunuwi
Location: Southern Thanalan somewhere near Qarn probably
Scholar-Hunter description:
The large population of golems in southern Thanalan is ascribed by mainstream historians to the mages of Belah'dia, who made the sturdy guardian for the protection of the Temple of Qarn.
Among these sad remnants of a civilization lost to time, Nunyunuwi stands out for his exceptional size and stealth.
At rest, he vanishes into the landscape of tors and boulders. Only a sizable battle will rouse the stone soldier to his ancient duty: to fight to the death.

There is a Fate spawning near the entrance of Qarn, where you have to kill several Golems to complete it if I'm not mistaken, so I think the spawn-trigger will have something to do with that and probably the amount of players participating in the Fate.
But that's just a speculation from my side >_<