Ah. You mentioned gods in the first one, so I naturally assumed that your second set of higher powers were gods as well.

My main concern with an unknown reason is that the way the different sides act towards each other would be fairly dependent upon what happened in the past. If it was a betrayal, the latent hostility is going to be a lot more extreme than if it was something that happened in a war. It would also be important to know how long it has been since they actually got along. Is it something that came about right at the creation of the world, or is it a more recent development? Since our characters are going to be on opposite sides, the way they would have viewed each other when they first meet is pretty important. Unless you want to pick general reactions, and later build a reason from that?