The most interesting alternative to the Planet Nine hypothesis (and to my mind a more attractive one) is that there is a self-organizing dynamical process that applies to large ensembles of bodies in highly eccentric orbits that causes their argument of perihelion to become correlated.

There is a great SETI talk by the astronomer Ann-Marie Madigan that explains the idea, it should be easy to follow if you've ever taken a mechanics course: https://www.youtube.com/watch?v=YW0Zb5gY0HA