So i was looking around in revendreth and noticed that the area where the worldege gorger spawns seems to be a secret venthyr research facility with a specific interest in devourers, perhaps they first lured the devourers in revendreth to study their anima proccessing capabilities and the situation grew out of hand or they did not really care about them destroying "lower" cities.
Could this be the reason for the devourers aggresive nature when we encouter them? It seems like they can be friendly after all since you can tame one and they defenitely have some santience. Makes you wonder what else Denathrius was doing behind everyones back.