lets keep this simple, a rogue planet, flung from its own star in a supernova, passes through your planetary system. the gravity of the planet is strong enough and it passes in just the right place to alter the course of your planet outside the gravitational influence of its own star. what now? well... it simply floats away i guess... goodbye planet.. we'll miss you :(...
now i see you mean to move two planets, one basic idea is an eruption of the star, in the hope that both planets are far enough away that they dont disintegrate. another idea is a phenomenon called a "white hole" on a small scale, its never been witnessed but is possible. it occurs when so much energy is forced upon some amount of matter that the matter performs a quantum warp. we know this is possible becaus in computers we have made the barriers between pathways for electrons so small that the electrons have been witnessed building energy and warping to the other side of the so called barriers. this isnt exactly a white hole but it proves the idea of a quantum warp. some scientists believe that black wholes do generate enough energy that when something falls into one it builds enough force to warp somewhere else, you could do a few things with this, you could have a white hole appear naturally and blast the planets away. or if your society is technilogically advanced enough you could compile nuclear energy from fusion in such a high amount and blast your planet into a warp tunnel. if your a halo fan you could think of it as sending your planet through slipspace.