There was a leakage path in the radio altimeter antenna system that caused the -8 feet indication. At some point on the approach, valid ground return would have been enough to overcome the signal strength of the leakage path. That's why erroneous radio altimeter indications don't show up at the gate, and require more than "black box swapper" skill to troubleshoot.
Did the #1 radalt start working correctly again at, say 1000 feet? If so, would the A/T have cancelled the Retard indication, or was it latched into a Retard logic routine? If the A/T logic had reset, the A/T would not have left the throttles at idle.
GB