When you change caster you are altering the steering "steering axis inclination" or the axis on which the spindle is turning. Adding caster brings this axis a little closer to parallel with the ground effectively making you feel it more in the steering when the suspension hits a bump. If this axis was completely perpendicular with the ground you would feel no steering movement at all on a bump but the car would be a unstable at speed. I would imagine this could be affected by scrub as well. Hopefully I explained this ok. This is also only relevant if ONE tire was hitting a bump independently of the other as they would cancel each other if say you went straight over a speed bump..