What causes bulging or herniated disc?

Aging, injury. Disc bulges are normal variants & not an issue that should cause a problem. This occurs with disc loading as well. A disc herniation is an actual disruption of the disc's coating layers allowing disc material to move out of a normal position. This is a more frequent finding as we age as the disc degenerates which is accelerated by obesity & smoking. Overloading a disc's weight capacity=herniation.