I believe that was explained in the Books of Sorrow. The Traveler chased the Darkness and cornered it onto Fundament. And was about to (I assume) eradicate the Darkness. But the Worm Gods seduced a young Aurash and her sisters. The Worm Gods enticed Aurash into believing the Traveler was causing the big disaster predicted to fall upon Fundament. And ever since then, Oryx and the Hive and the Darkness have been chasing after the Traveler