Zygarde is known as the Order Pokemon hence the reason why it stops the boosts of Fairy Aura and Dark Aura as you can easily see it as mutating what once was as boosting the power of 2 specific types can be seen as distorting the natural balance.
Life and Death are 2 different mysteries which despite their differences are the source of ones existence for one does not live without not knowing of death. However to accomplish this there must be Order in order for the both to activate each other.
You could also look into the goggles in which Fairy Aura brings life and Dark Aura takes away
If this should happen the balance between life and death would fall causing an intergalactic breakdown and a hole in existence
With Aura break stopping this disorder, Zygarde is like I mentioned above the bringer of order
It's classification was confirmed here