gravity of all the planets pulling on each other pull them into a stable, flat plane. Imagine all the planets but one rogue one are in this same plane, like circles on the surface of your desk.. The rogue planet orbits at a big angle, so even if it is going in a circle, half of the time it is above the desk, and half the time below (like this: https://upload.wikimedia.org/wikipedia/commons/thumb/e/eb/Orbit1.svg/1280px-Orbit1.svg.png - the grey is the plane all the planets are in, apart from the rogue one which is the yellow orbit) . When it is above the plane, all the other planets are located below it, and pull on it with gravity, down towards the plane. When it is below the plane, the planets in the plane are all above it, and they all pull it up. Over a very long period of time this brings the rogue planet closer and closer into the plane. Once within the plane, the planets still pull on each other, but there are no forces which would cause them to leave the plane again. Over time everything will settle into this single plane as its the only true long term stable solution.