The GCU's control the BTB's and GCB's, but the BCU's (Bus Control Units) have control over which power sources actually go on-line. The #1 BCU is essentially the master, with the #2 BCU acting more as a slave rather than a backup. Sounds like it was the #1 BCU which stopped the show (as mentioned, this was one of the units replaced after the incident).
There were no dark and mysterious human factors at work here (other than in the original cause of the problem).
If it's not already written in the manuals, IMHO, it would be better to manually select the Standby Power Switch to Battery instead of leaving it in AUTO, especially if the GCU's are not able to supply a constant/reliable source of AC to the busses. Constant tripping and setting of relays is going to cause extra problems.
Rgds.