Correct. Square-Enix has never talked about how any NM is triggered (and only occasionally releases general information about other related topics, such as BCNMs and ENMs - usually just the bare minimum amount of information necessary for players to attempt it - see recent discussions about the formulation for Goblin Drink for an example of just how obscure S-E has been in the past).
The trigger for this NM is unknown.
However, I am seeing speculation
here that it might have something to do with Apkallu hate levels. While there's no confirmation, it seems plausible - it's known that Apkallu have an enmity system very similar to what Fomors and Tonberries have, so perhaps you need a lot of Apkallu hate to get him to spawn for you.
Icemage