User blog comment:LastationLover5000/Fairy Tail 301 - In Which Hiro Has Been Reading My Things (The Dragon King)/@comment-3061966-20120928205851/@comment-4666155-20120928213330

Sadly, all we can do is wait. There is a high chance a dragon transformation could be natural, while Acnologia's is due to Zeref. My theory would be an overuse of Dragon Force, which makes a person even more like a Dragon than learning Standard Dragon Slayer magic. It makes sense to me anyway