When the tilted field of Uranus was first discovered, one interpretation was that the field was measured during reversal. When Neptune's field was also discovered to be tilted, however, the chance of catching two fields in the process of reversal was thought to be highly unlikely, and this hypothesis was rejected. The displacement of Neptune's field from the center of the planet is actually a clue to its origin. However, the displacement of Neptune's field suggests that is may originate from near the surface of the ice. The hypothesis has therefore been made that these fields originate in a layer or shell of liquid water at or near the top of the ice in Uranus and Neptune.
Water is a conductor whereas ice is an insulator.
There are probably many competing self-generating magnetic dynamos in this water layer or shell, some stronger than others, which is why the resultant field is tilted relative to the spin axis. These speculations are based on magnetic data from one fly-by of each planets. Atmospheres, Rotations, and Rings All of the Jovian planets have relative rapid spin rates. Jupiter and Saturn rotate on their axes about once every 10 hours, which is very fast considering their great size. Uranus and Neptune rotate with -18 and 16 hour periods, the negative sign for uranus indicating a retrograde spin.