How exactly does a disk become herniated?

Wear and tear. A herniated disc occurs when the outer part of the disc weakens allowing the inner fluid filled nucleus to bulge out. It may occur centrally, laterally or both. If the bulge is large or other factors exist, pressure on the spinal cord or a spinal nerve can cause pain, numbness and weakness. The cause is usually wear and tear of the spine, injury and repeated micro-trauma are examples.
Let me count the way. This can occur due to natural aging, related to a spine fracture, due to a lift, twist or bending injury, from a disc space infection, from a spinal instability or deformity, related to a smoking history in terms of smokers having accelerated degenerative changes & as a result a 2x higher rate of disc herniation compared to non smokers, & there may be a genetic or inherited basis for this as well.