Without access to the wiring diagrams, or knowledge of what systems are fed by each generator bus, I haven't got a clue. I merely mention this as a possibility. In the absence of reliable flight instrument readings controlled flight would be very difficult at night. While the flight instrument panel may run from the battery supply, it might also take several seconds to switch over in event of sudden generator failure. This is all pure speculation, but it would account for loss of control by an experienced pilot and point the finger of suspicion at purely technical causes for the crash and not any human failings.