Soon to know the 777,
A little auto land history,
Auto land had first required a single A/P with a monitoring redundant channel verifying input and output channels. It has always been a single flight control /A/P function with a second or third channel verifying the data to approve 3A or /B for a minimum dual land.
Good educated bet is that one input to the double or triple redundant system failed. Most likely ILS/MMR or R/A glitch or failure.
Bottom line is that most modern aircraft, hell even the 727 is capable of a 3A landing on one autopilot or servo in this case. All aircraft I know of (commercial) only use the other A/P channels for redundancy.