Paraphrased from the Dragons of Faerun sourcebook:
Long before the Crown Wars, there was the Age of Dragons. Dragons ruled over all other races (their foodstock) with impunity. The elven high mages, working in secret in the northern glacier, weaved a giant mythal over all of Faerûn: The Dracorage Mythal.
This mythal would induce insanity and rage upon the dragons, causing them to attack and destroy anything in their path. This included their lairs, offspring, followers, anything.
Reduced to mere rampaging beasts, the dragons could no longer rule absolute over all other races; they had won a fighting edge over the dragons.
The dracorage mythal is linked to the appearance of the King-Killer Star. Depending on the position of the comet in the sky, the rage can be localized or global.
In 1373 DR, the link between mythal and comet was broken:
Hammer 1: Sammaster successfully ties his phylactery to the Dracorage mythal, separating the Rage of Dragons from the King-Killer Star and effectively causing all dragons to go mad. Sammaster is later destroyed (on Nightal 6), ending the Rage of Dragons.
The dracorage mythal still exists, but now the only way to trigger a rage is by using the extremely rare 4th level spell Aggravate Dracorage. This spell only exists in 3.5e though.