How did I not see this earlier? That's caused by the black hole's gravity well, and is (mostly) intentional.

I think the volcano planet should be moved a little farther away from the black hole (unless you intended it to have it close to the black hole to make it unstable)

It's not a bug, it's a feature. I wanted the black hole's gravity well to be able to pull things offworld, the idea being that the extreme tidal forces are slowly tearing the planet apart. This is also why the black hole is surrounded by an asteroid field, it's debris either torn off the planet or from a destroyed moon.