This is usually caused by a slight delay during RSS startup and your planet being placed very close to the origin at 0,0,0. If you have a lot of planets or a slower computer there is a small delay where proxy worlds are all loaded in at 0,0,0 until they are moved to the correct position. During that time, gravity still works normally, so if your planet is too close to the center unparked grids are pulled toward the center by the combined gravity fields of ALL of the planets and stars in the system. Just how far away they need to be depends on the size and gravity of the planets, as well as their proxy scales.