The European grid is a connected synchronous grid. Usually that would add stability but it also means if a single country's grid blacks out then their neighbours have to respond to that. Portugal and Spain's grids will be intimately connected which won't have helped.
The whole European grid is essentially connected at this point, it's a simple trade of resiliency for efficiency. People just forget about the resiliency part until something goes wrong
Sounds like a major infrastructure risk given that it is possible for more than one country to experience a full loss of power.
EDIT: Andorra is also affected, so that is three.
[0] https://www.lavanguardia.com/vida/20250428/10624908/caida-ge...