There can be a multitude of reasons why demand cannot be met. During storms facilities can go down too, any number of possible failures affect their ability to run.

If you run gas plants you need gas and water, if water isn't available you can't run. If a switch fails because it's frozen you can't get your power out, if gas supplies are poor you can't run.

If demand exceeds your spinning capacity you need to cut back, rolling blackouts can prevent total collapses so they will do that. Most places have plans in place, heavy industry is supposed to cut back first and then they'll roll blackouts here and there. That way no one is without power very long.

Sometimes it is simple red tape and government foolishness which causes blackouts, sometimes it's poor planning, equipment failures, operator error (that happens a lot), trees on lines, ice on lines, strong winds even.

We had turkey vultures shut us down several times, someone shot a hog with a bow but didn't kill it and it ran off under one of our lines to die, vultures were sitting on our lines as they took turns eating the hog. Their wings shorted us out, tripped off 138000 volt circuits. We couldn't chase the birds away because they are protected...we ended up having to rerun our lines, spaced them out so the damn birds couldn't reach across and short them. Those freaking birds cost millions of dollars...

A lot of legit reasons why power might trip off or not be available, it is usually a tangible reason ho, not just because some manager is a richardhead.