The similarity between that event and this early-on report is striking.
[es language]: https://www.lavozdegalicia.es/noticia/espana/2021/07/24/aver...
I suppose it makes sense that it was an automatic shutdown rather than infrastructure failing on such a wide area. And then once it's shut down, a black-start is a logistical challenge as other comments have explained.
I'm also seeing some reports about it being more likely that something happened on the east side, somewhere like the Ebro valley or north across the Pyrenees. Catalonia seems to have been particularly affected, and it's on the path of important lines coming from France. High heat at noon could have caused a line to fail and short against a tree, which would be similar to the 2003 nation-wide outage in Italy.
In theory [a flawed one] you've had enough spare capacity to survive N failures and N+1 failures are statistically unlikely because p^(N+1) is close to zero.
On practice [or with a better theory] you can't multiply probabilities in a grid system because random variables aren't independent. 30% spare capacity can go to -100% in a second.
"Le gestionnaire français souligne par ailleurs que cette panne n’est pas due à un incendie dans le sud de la France, entre Narbonne et Perpignan, contrairement à des informations qui circulent."
Called N-1 criterion. See https://en.wikipedia.org/wiki/Contingency_(electrical_grid)#...
And it depends. During https://en.wikipedia.org/wiki/2006_European_blackout N-1 criterion was supposed to be holding, in practice not even N-0 was holding and network crashed.
Few years ago nearly entire day European network was sitting on N-0 due to multiple issues in Poland, caused by a heat wave and deeper root causes. There are many power plants and power lines where any further issue would cause Europe-wide blackout.