History of the Drow, The Dark Wars:

The warlike drow nature did not change when they escaped their surface foes during The Descent. In fact, they immediately launched a series of wars to establish territories in the Underdark. They began by stealing and seizing dwarven magical items, and using them against the dwarves - establishing an enmity that is still strong today.

The drow then fought among themselves, noble against noble, priest against priestess, for rule of their new realm. This all-out war ended amid great magical explosions that brought down the roof of the largest dwarven cavern they had seized, great Bhaerynden. The ceiling collapsed entirely, burying many drow and the shattered dwarven cities. The cavern, now open to the sky, became known as The Great Rift. The surviving drow nobles gathered what people, slaves, and equipment they could seize, and fled into the Underdark in search of places to dwell. "The Scattering" brought about the many rival, self-interested cities where most drow live today.