I don't think we're able to tell from the data if one is the cause of the other, are we? Since if production was lost, load would have to be shedded to balance the grid, and if load was lost (e.g. due to a transmission failure), production would have to be disconnected to balance the grid.
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)
Renewables were a factor in the blackout here in Brazil a couple of years ago: the models used by the system operator did not correspond to reality, many solar and wind power plants disconnected on grid disturbances quicker than specified. That mismatch led the system operator to allow a grid configuration where a single fault could lead to a cascade (more power was allowed through a power line than could be redistributed safely if that power line shut off for any reason), and that single fault happened when a protection mechanism misbehaved and disconnected that power line. The main fix was to model these solar and wind power plants more conservatively (pending a more detailed review of their real-life behavior and the corresponding update of the models), which allowed them to correctly limit the power going through these power lines.
If you want an excruciating level of detail, the final 614-page report is at https://www.ons.org.br/AcervoDigitalDocumentosEPublicacoes/R... (in Portuguese; the main page for that incident is at https://www.ons.org.br/Paginas/Noticias/Ocorr%c3%aancia-no-S...).
If you have a large spinning inertial mass like a factory motor or a power generation turbine, it's extremely important. Imagine a manual car transmission, but there's no slip-clutch, you need to perfectly align engine with the wheels rotating at 300mph, and the inertial mass you're up against if it's not perfectly synchronized is a freight train.
That's why generators trip offline in a blackout cascade if the frequency deviates out of spec. The alternative is your turbine turns into a pile of very expensive shiny scrap metal.
Frequency coordination is absolutely critical, via phase coordination. A large generator must not get significantly out of phase. So frequency going out of spec triggers the generator to "trip" (disconnect).
Its like a three legged race. You and your partner have to run in synchronism. If either of you slows down or speeds up, the other can trip and fall over taking both of you out.
if a high power transformer goes out of tune... it melts or blows up (or both); it'll try to shut itself down before that. getting it back on becomes a problem if other transformers do the same thing, which is apparently what happened in the whole country.
In reality, power generation equipment will disconnect itself if the frequency is too low/high to avoid catastrophic failure.
