If they are Lusa tribe, then the reason for the attack could be to get rid of "unnatural" energy since the Lusa view unnatural stuff as an enemy and the dragons are immune to natural energy attacks (which implies that Rato energy, which could hurt dragons, might be perceived as unnatural).
@FStubbs I was actually thinking about the possibility of the great war being the same war mentioned in Claymore, these dragon dudes are the same ones that were used to create yoma, and that the people who made photon carriers also created the organization. I think this might be a story about the mainland years after the war had been resolved. It's more likely that Norihiro just liked the ideas he had in Claymore and decided to use them again them for a different kind of story, but I choose to believe it's connected until I'm proven wrong.