Question about intervertebral discs. why do they usually herniate in the posterolateral direction?

Weakness of Annulus: Posterolateral herniation occurs most often because the most common biomechanical forces including rotary and flexion loads force the nucleus pulposus to herniate through the annulus fibrosis in the posterolateral direction, and is also related to the elastic modulus and weakness of that annular segment due to repeated mechanical loads that have weakened it in that direction. See link below.