air

I forgot the 4th and 5th factors: the uneven suspension damping between left and right fork legs and aerodynamic lift. 
As to the forks, this is what contributed mightily to my Triumph's stability issues. I had 10mm more preload in one leg than I had in the other. At 80mph even with 50lbs of luggage on the back it was no problem. Just steered a little odd - left was easier to initiate over right. With the shock not sufficiently preloaded for the bag contents I was running nose-light which set up and then quickly escalated the head shake once aerodynamic forces had built up enough.
 
But back to the FJ, when you put the damping all in one leg, the other one is able to move faster. It is the front axle alone that tries to keep things in sync. But front axles flex, there is bearing slop, and minute torque moments introduced every time the suspension moves.