Orcus is a plutino, locked in a 2:3 resonance with Neptune, making two revolutions around the Sun to every three of Neptune's. This is much like Pluto, except that it is constrained to always be in the opposite phase of its orbit from Pluto: Orcus is at aphelion when Pluto is at perihelion and vice versa. Because of this, along with its large moon Vanth that recalls Pluto's large moon Charon, Orcus has been seen as the anti-Pluto.
Orcus and Pluto have "mirror image" orbits, and are both tied to Neptune's orbit. Then it struck me that that makes them like Jupiter's trojan satellites, or the asteroid that is in a horseshoe orbit around Earth.
Pluto just happens to be in a gravitational "sweet spot" with respect to Neptune. Orcus is at another. That gravitational sweet spot also explains why Pluto has five moons, and maybe even rings. Pluto is just a piece of dirty ice that got caught in a gravity divot. Same with its moons. Apparently, all of the other Kuiper belt "planet candidates" are also in gravitational resonance with Neptune.