The Great Calamity
The Great Calamity was a nearly world-ending war between the dragons and the gods. Very little is known among other races of the cause or the time before it, as the dragons burned the world in their rage and destroyed most civilizations at the time. There are still traces, though, and the Birdfolk have made it their mission to understand what happened.
What little is currently known by other races is that some kind of event sparked anger so great among dragons that they, as a race, took it upon themselves to destroy what the gods had created. And for a time, they succeeded. The other races survived by hiding deep underground and sending someone once a month to see what the outside world was like. Most often, they didn't come back. And when they did, it was with tales of fire and lightning.
One day, a terrible howling was heard across the entire continent, as if the world roared in pain and loss. And then, all was quiet.
When the races began to venture out after a time, there were no dragons in sight and they could see that the land was slowly starting to recover. The rare few who did see dragons reported them moving south, across one of the rivers.
In time, the birdfolk discovered a little of what had happened: the gods had cleaved the dragons' tongues and took their language from them. The Dragon Tongue was lost both to dragons and the other races and the powers of magic were much weaker for it.
However hard anyone pressed them for information, the dragons of that time would never speak of what happened during the calamity. Even today, dragons are forbidden to speak of it. They will only say that the matter is still unresolved, but that they have to live with what happened and hope that one day they have the chance to speak their language once more.