HJ,

In fact we know quite a bit about polar wandering and polar shift. MoBOB was somewhat correct, precession does have something to do with earth movement, but that movement is much slower than the wobble introduced by the fact that the earth is a slow gyroscope (think of a top as it slows down). The convection cells in the outer core (where the magnetic field is produced) are also wobbling, not necessarily in the same sense. The motion of the two of them together might be in sync (slow polar wandering) or out of sync (fast).

As for the reversals, they are probably not sudden - at least in human terms - but take several thousand years (if some modelers are correct, we might be in the beginning of one) and once again are introduced by changes in the circulation of the outer core.

Respectfully,

Jerry