It(Uranus) rotates on its side, tilted almost 98 degrees from the plane of its orbit around the sun. The axis of its magnetic field is tilted too, at a 59-degree angle from the rotational axis. The magnetic field is also off-center, with the field lines emerging about a third of the way toward the south pole. (source)
Neptune's magnetic field is tilted 47 from the planet's rotation axis and is offset at least 0.55 radii, about 8,500 mi (13,500 km) from the physical center. The dynamo electric currents produced within the planet, therefore, must be relatively closer to the surface than for Earth, Jupiter, or Saturn. Because of its unusual orientation, and the tilt of the planet's rotation axis, Neptune's magnetic field goes through dramatic changes as the planet rotates in the solar wind. (source)
So, it is well established that the magnetic axis of Uranus and Neptune is off-center and due to this, the planets faces the following consequences:
- the magnetosphere is irregular
- the strength of magnetic field varies, almost opening and closing periodically as the magnetic field lines disconnect and reconnect
- the orientation of magnetic field changes constantly.
But, why is the magnetic axis off-center in the first place? I found no source answering that. Is it due to alignment/orbit/internal heating mechanism of the planets?