Maybe there needs to be some form of balance of power between Hydaelyn and Zodiark, Equality to maintain the equilibrium of Light and Dark. But with Hydaelyn being weakened due to her direct intervention during the Ultima battle, it allowed Zodiarks power to increase thus throwing the scales of balance even more out of alignment.

As a numerical example of the point I'm making. Say both Hydaelyn and Zodiark are at 100 power each. Balance is maintained. But Hydaelyns power dropped to 50. In that case Zodiarks power will increase to 150 due to that "energy" not being able to disappear. Science 101, energy cannot be destroyed.