So that's not entirely accurate. Changing the caster can raise/lower the location of the end of the steering arm, but it doesn't necessarily make the bump steer worse or "cause bump steer". Depending on the ride height of the car, suspension components, and the rest of the alignment specs adding caster may actually improve bump steer.
Changing the caster can alter the bump steer profile, but whether it makes it better or worse depends on how the suspension is set up and the rest of the alignment. It's not as simple as "you added a bunch of caster, therefore you have bump steer".
Any time the ride height changes the alignment specs change. That's true of all suspensions, it's a little more of an issue with torsion bar suspensions just because you can adjust the static ride height more easily.
The alignment we set is the static alignment, the dynamic alignment numbers are always changing when the wheel travels up and down. That's why camber curves come into play, where bump steer comes from, etc. Remember that bump steer is just the toe change curve, the toe you set at ride height is not the toe at full compression or extension.