What Caused the Kuiper Belt to Get Warped?

"Imagine you have lots and lots of fast-spinning tops, and you give each one a slight nudge. If you then take a snapshot of them, you will find that their spin axes will be at different orientations, but on average, they will be pointing to the local gravitational field of Earth... We expect each of the KBOs' orbital tilt angle to be at a different orientation, but on average, they will be pointing perpendicular to the plane determined by the Sun and the big planets."

"But going further out from 50 to 80 AU, we found that the average plane actually warps away from the invariable plane. There is a range of uncertainties for the measured warp, but there is not more than 1 or 2 percent chance that this warp is merely a statistical fluke of the limited observational sample of KBOs... The observed distant KBOs are concentrated in a ring about 30 AU wide and would feel the gravity of such a planetary mass object over time, so hypothesizing one planetary mass to cause the observed warp is not unreasonable across that distance."