UPS systems and generators fail. In the 2000s probably the worst 2-3 days of my career happened when our colo provider lost power in their data center. They had redundant UPSes and a backup generator but something went wrong in the cutover and their entire facility lost power. We were a tiny company and did not have a lot of protection against this sort of failure -- that part is certainly our fault -- but the fact is that something that you wouldn't expect to happen, happened.

Granted, Delta is not a tiny company and has infinitely more resources to throw at the problem than we did. But the takeaway is that even redundant failover, sometimes fails.