Most people I spoke to around Maresme area yesterday believed someone incompetent made a mistake somewhere, and that was the reason. Some people jokingly suggested cyber attack/Russian probes, but afaik no one took those seriously.
There was actually a Russian cyber attack on the previous friday, pinging some German city government sites to death. Dresden, Nuremberg and one other. But this was essentially kids play, compared to a nationwide power blackout. A cyber attack to bring down a nation's grid would be high sci-fi. This would need lots of explosives and expensive physical interventions. Russia didn't manage in Ukraine eg.
On the upside, more people at all levels are going to be more prepared if it happens again.