Recent models for the formation of these two planets have come up with some hypotheses as to how the strange field geometries of these two planets came about....
The phase diagram of water and the magnetic fields of Uranus and Neptune
Numerical dynamo models of Uranus' and Neptune's magnetic fields
Dynamic Ionisation of Water Under Extreme Conditions
Weird Water Lurking Inside Giant Planets
Superionic Water
What they have hypothesised is that the magnetic fields of these planets are not being generated in the cores of the planets but in the flow of hydrogen ions at very high temperatures in the mantles of these two worlds. The hydrogen ions occur in a substance known as "superionic water", which is essentially a type of "ice" that occurs at high temperatures and pressures underneath the atmospheres of the two planets. The oxygen ions in the ices form a lattice like structure through which the hydrogen ions (stripped away from their original water molecules by the high pressures) are free to travel.
It's essentially a partially degenerate phase of matter somewhere between a solid and a liquid in nature...acting much like a metal.
This flow, because it is occurring at a distance from the core, means that the currents being generated that create the magnetic field of the two planets are substantially offset from the axis of rotation of the two planets. The geometry of the planet's magnetic fields are therefore shaped by the offset position and the rotational dynamics of each of the planets.