From experience elsewhere (not being able to see behind the curtain at Delta) I'd hazard a guess that their IT guys have been beating on the management for years to get proper hot fall-back for what is nowadays a mission-critical system, but management (supported by bean-counters) have been stalling on the necessary investment.
OTOH, they may have all the fall-back in the world but they've never actually exercised it properly, so when it's called on for real, it doesn't work ...