Plainly the reversers did not deploy. They either unlocked or the system sensed they were unlocked when they weren't.
There is a prox sensor on the lock, and another at the top left of the cowl. If either is 'far' then the amber REV will come on. It is extremley unlikely that the reverser unlocked. Most probable is that the cowl closed sensor went to 'far'. I have had this happen when a flex drive sheared between the motor and the number 1 gearbox. The reverser doesn't move, but the top left corner of the cowl flexes just enough to bring on the REV indication. Unfortunately at present on a B744 this gives a signal to the inboard l/edge flaps. I expect a mod to come out to need the unlocked sensor to indicate 'far' as well.