The DWP says that workers cut one bundle of wires, and that this resulted in a cascading series of shorts and surges that took out half the city plus all of Burbank and chunks of Glendale as bits of the system protected themselves by shutting down. But the system "is not fragile", according to their big cheese. I just run the infrastructure for a big website, but if one cable cut could take down the site like that I would offer my resignation. You buy two cables, you see. Two!
Is it that difficult to prevent these cascading shutdowns? In 1996 an interstate power line failure caused half the West to go dark. One would think that the automatic overload prevention systems would have been worked on a bit in the last decade.