Complex sentence: The orbits of objects that are metastable, for example some that are initially aligned with Planet Nine, precess until parts of the orbits are tangent to that of Planet Nine, leading to close encounters.
Split into simpler sentences: The orbits of some objects are metastable, for example some that are initially aligned with Planet Nine. These orbits of these objects precess until parts of the orbits are tangent to that of Planet Nine, leading to close encounters.