Somatogravic Illusion
I’ve experienced it once at night and I can tell you it is almost totally overwhelming. I can easily imagine the FO (due to the large acceleration) believing he was pitching very high nose up and ‘instinctively’ pushing forward. That increases acceleration which makes the illusion even worse, so you want to push forward even more. By this stage he would have been in cognitive overload and totally unresponsive to the CA’s desperate attempts at recovery.