Lets consider only the moment before the collision of the small planet pieces: these are all accelerating to the center of mass. If you revert time, you still h

Lets consider only the moment before the collision of the small planet pieces: these are all accelerating to the center of mass. If you revert time, you still have a lot of pieces, with high speed, but these are going away from the center (and getting slowed by the gravity). After collision, it gets more tricky, because the kinetic energy of a body will be lost (mostly transformed in thermal energy) and it will be less intuitive to imagine how to revert that.