Mentioned it in the Healer forums, so I'll bring the quote up here:

Quote Originally Posted by FeliAiko View Post
Possible reason behind the choices could be due to the Class/Job's starting City State and the associated Primal. Therefore Ifrit for MNK (Ul'dah), Levi for NIN (Limsa), and Garuda for DRG (Gridania). It also reinforces the existing separation between the melee roles by giving them a Primal from a different City State to each other.

With that in mind, they may assign Ramuh and Mog to the remaining Gridania Jobs (WHM/BRD). Tanks will most likely get Titan as that thematically suits them more than any other role, which in turn left Shiva as the only one left for BLM.