Pressure and. Movements of the spine. Generally speaking the more you twisting and bend at the waist while holding a heavy object the more likely you will result in a bulging or herniated disc due to the pressure inflicted on the area of the spine.
Herniated disc. An interesting question and many answers but in short the disc will herniated if too much pressure is placed on it such as in trauma. The other cause is extensive wear and tear on the spine leading to disc degeneration and weakness in the ligament around it leading to it to herniate - plus a number of of less likely causes.
Most commonly. it is a result of usual degenerative process. Where slowly the turgor of the disc is slowly lost, due to decrease in the amount of hydration in the disc. This results in disc space to lose height, then bulge out, which puts extra stress on the disc covering (annulus), which can slowly have cracks through it, and then the disc within will make its way out through the annulus, and herniate.