why does the headrest manipulate the spine?

Augustus

No, I think that's exactly what it is.

I've owned Saabs and Volvos that made a big deal about having active head restraints, and those did exactly what the OP is describing. If you pushed back on the headrest, you could move the mechanism inside the seatback and feel it on your upper back.

The idea is to minimize whiplash. Someone rear-ends you, your torso is pushed into the seat like you're accelerating, but obviously much more suddenly & aggressively. Active headrests use the inertia of your upper torso to pivot the headrest forward to help cradle your head in this type of accident.

If the Z has active headrests (anyone?) this is what the OP is feeling.

A couple of quick images illustrate how they work..

Volvo:


Saab: