From the one line we get regarding the Warriors of Darkness' reason for formerly opposing the Ascians, it sounds like they just puppeteer history on the Reflections.

Quote Originally Posted by Arbert
This day we reclaim the reins of history! This day we rid ourselves of the Ascians forever!
They would likely do this to maintain relative balance, lest a Reflection become drowned by Light or Dark and become a Void. We know so little about the Reflections that there isn't much to do but guess at this one, lol. Maybe they trigger Calamities on the Reflections... but I can't fathom why they'd do so, given the Reflection needs to survive to have its Shard Rejoined. (Again tying into why they don't want Voids of either flavor.)

Lahabrea mentions the aetheric imbalance, though what exactly he means remains unknown. He makes it sound like a natural disaster will occur if the aetheric imbalance is not mended by complete Rejoining, and may be correct; the Light seems weak on the Source, but we never do anything but banish the Dark. Elidibus also mentions the balance is tipping toward Light in the ending of Heavensward, so they may be right... though what exactly that means is the question. (I have no answers because there's still no evidence to draw a conclusion from; we now have most of the "how" behind the Ascians' actions, but the ultimate "why balance the Source?" remains.)