Planetary migration
Planetary migration is the act of a stellar satellite altering its orbital parameters, especially semi-major axis, through various means during its lifetime.
Core Accretion
During the core accretion phase, a gas giant is thought to usurp angular momentum from its surrounding dust cloud such that it falls inwards towards the primary.
Late Bombardment
During this phase of planetary creation, planetessimals gravitationally interact in a chaotic manor causing many to be cast off into new orbits.