Any process that allows a planet to exchange energy and/or angular momentum with another component of the system will lead to migration. The fact that we know migration must happen stems from the inconsistency notes between planet formation models and observations of planetary systems. Most models do not allow giant planet formation at locations such close to inner regions of the disk where we have observed exoplanets. Thus mechanisms must be in place which alter planetary orbits.
In gas disk migration, that is, the planetary migration which happens through interaction which gaseous material in the disk, is mediated by gravitational torques between the planet and the disk.