Interacting time varying magnetic and electric fields must be the cause, as used to generate electric currents with generators in your car. This Dynamo concept is due to Faraday. Even without all the computer power you can understand a few key features of the field generating process. From Maxwell s equations, and assuming a velocity field in the core, you get the kinematic dynamo equation. This equation is linear in the magnetic field, so if a positive field is a solution, so is a negative field that is a polarity reversal of the main field does not require stopping all the fluid motion and starting it up from zero.
Some stirring mechanism is required to keep the fluid moving.
Radioactive minerals may not exist there. Instead, it is now widely accepted that gravitational convection, settling of a precipitate of pure iron accreting to the inner core is stirring the fluid outer core as it forms, so that the inner core grows, the outer core is stirred, and over the age of the earth, the core will eventually freeze and the magnetic field will disappear. The main field varies over a variety of time scales, from secular variations in magnitude and direction over years to complete reversals apparently at random. We think the core is highly conducting so that magnetic field lines are frozen into the core material it takes about 10,000 years for the core field to completely collapse, even if the energy source were turned off. This effect, plus the shielding influence of a conducting lower mantle mean that the field takes years to change appreciably.