Somatogravic illusion may have been at play here.
AP disconnects due to IRS failure; this failure generates ambiguous nose up display on P1 PFD. P1 pushes ND in response and aircraft accelerates. Both pilots now have sense of increasing pitch up. P1 pushes more as PFD is still showing pitch up. P2 'feels' pitch up but sees nose down on PFD but doesn't comment as they have the PFD miscompare - he may believe his in error. Only when they bring in the performance instruments do they realise something is wrong but as has been mentioned, the aircraft is now very poorly placed.
Pure conjecture, but dark nights with no visual references have caused aircraft to crash due to pilot disorientation even with perfectly serviceable instruments.