Reuters is already reporting the likely source of the outage, opening the door to finding cause.

REE’s system operations chief Eduardo Prieto told reporters the loss of power supply was beyond the extent that European systems are designed to handle and caused the Spanish and French grids to disconnect, which in turn led to the collapse of the Spanish electricity network.

On the 25 of April, just days before the outage, Spain had called out security risk with the French interconnection.

…Joan Groizard, Spain’s secretary of state for energy. Better interconnection with the rest of Europe would increase energy security…