When a disc is said to have ‘migrated’, this means that it is no longer herniated at the level of the disc. Instead it has sequestrated and traveled beyond the margins of the normal disc and is not sitting behind the upper or lower vertebral bone. Discs migrate because there is a lot of pressure from all the weight and forces acting on the spine.
Therefore, when there is a tear in the outer disc, the inner jelly-like material (ie. Nucleus pulposus) will herniate outwards. However, it can only herniate so far before it encounters nerve, bone, or other ligaments. As a result, it then has to either travel downwards or upwards, and is then said to ‘migrate’.