Report on a smaller event in the UK: https://www.ofgem.gov.uk/publications/investigation-9-august... (see linked pdf)

That started from a combination of a lightning strike and generator trip, but turned into a local cascade failure as lots of distributed generation noticed that the frequency was under 49Hz and disconnected itself. I suspect the Spanish situation will be similar - inability to properly contain a frequency excursion, resulting in widespread generator trips.

(I suspect this is going to restart a whole bunch of acrimony about existing pain points like grid maintenance, renewables, domestic solar, and so on, probably with the usual suspects popping up to blame renewables)