There's a few quotes from Alberic, the previous Azure Dragoon, that I feel sheds a lot of light on the conflict.
Which ties into the following.Originally Posted by Alberic
Obviously it's all just a load of speculation at this point, but I feel like the Dragonsong War is ultimately a thousand year vicious cycle. At this point, we know the dragons were wronged by the Allagan thousands of years ago. Both their supposed god and they themselves were enslaved and forced to bend to the Allagan Empire's will. (I.E Twintania, and her mind control device.) So the mortal races already set a precedent back during that era.Originally Posted by Alberic
I feel like we're going to find out the Dragonsong War started because of a misunderstanding, and has simply escalated because of vengeance on both ends until it became a never-ending vicious cycle. As was mentioned earlier, a situation where both sides have justifiable means to their ends, however each side only serves to further the conflict.
It seems like, from Alberic's quote, the dragons themselves are suffering. I doubt they would willingly bring that on themselves. It's an interesting subject, but we really can only take shots in the dark at this point in regards to what started the war in the first place. Based off the lyrics from the Heavensward trailer, and those that Ferne teased us with from the Dravanian's perspective, I feel like it might have be something akin to the Ishgardians stumbling across the dragons at some point, which startled them as much as it did the Ishgardians, which then led to an initial conflict that spiraled out of control.
/tinfoil hat
If we want to go on a more colorful route of crazy theories that are pulled out of thin air and have next to no real basis, I'm partial to the idea of these floating islands that we'll be visiting in the expansion being either the "home" of the dragons, or a place of great significance to them. Maybe the Ishgardians saw these floating islands at some point, and thought they were the heavens (Heavensward?) where the twelve (Or Halone, specifically) dwelt, and decided they were going to reach them somehow. Maybe they eventually did, and sought to claim them as their own, only to realize they were home to the dragons, who had hid up there to avoid the mortal races after their less than awesome experience with the Allagan Empire. Possibly even a place to "slumber", as mentioned in the Heavensward lyrics. At least until the Ishgardian people barged in and ruined everything.
To take things one step further into the crazy, what if Heavensward is actually in reference to a "Heaven Sward", as opposed to a Heavens Ward. A sward being an expanse of short grass, or in this case, a large field on top of a floating island.
/tinfoil hat off