Spanish grid operator (Red) preliminary Engineering Report seems to be blaming renewable generation (solar) for the initial drop followed by second drop 1.5 seconds later.
> REE's System Operations Chief Eduardo Prieto told a news briefing the electricity system was hit by a dramatic power generation loss in southwestern Spain, that caused instability in the system that led to its disconnection from the French grid. He said it was quite possible that the affected generation was solar, but it was too early to say for sure.
This doesn't quite read like them blaming solar either. If anything, they're painting it as one of the victims of the failure.
[0] https://www.reuters.com/world/europe/spanish-grid-operators-...
1. System was running with low inertia due to a high amount of renewable generation (mainly solar).
2. Something caused a large amount of generation to trip. Lack of inertia in the system can't arrest the frequency drop.
3. Rate of change of frequency (ROCOF) protection kicks in and trips renewable generators; interconnectors to France also trip as they are suddenly overloaded. Islanding and blackouts ensue.