In a nut shell, as you travel over a bump in the road at a high rate of speed the up and down movement in the suspension causes the wheels to turn left and right making the vehicle change direction. This happens because the geometry changes even though you were holding the wheel straight! By removing all but two springs, the up and down travel of the axle is now way greater than the steering was designed to handle which in turn amplifies the bump steer.
Toe-In is what helps keep the truck running straight when you take your hands off the wheel. It also helps automatically return the wheels to straight after making a turn.
Mike B
