It can be us, using too much power all of a sudden (like when we all come home after a long day’s work and simultaneously turn up our air-conditioning just as the wind slackens), or it can be cloud cover, stripping the generative capacity from solar panels. Regardless, the utilities and other balancing authorities have to act very quickly to set things right again. Otherwise there just isn’t enough power in the lines to keep the lights on. Lots of blackouts start this way.
