It basically boils down to a geometry problem that has to do with the angle of the king pin and how that angle applied down through the the spindle intersects the wheel bearings on the spindle. A dropped spindle changes the angle and moves the bearings away from that angle causing the load of the front end to shift in relationship to the spindle and wheel bearings. This will put more strain on the bearings.
A similar problem can also show up when using reversed wheels or wheels with an outward offset. Usually the first thing to go in this case will be the outside wheel bearing. It may take awhile, but it will eventually show up.
I've learned to carry en extra set of wheel bearings onboard.
