Or one big solar flare.
I’m a lot more skeptical of a CME or solar flare causing the kind of geomagnetic currents to take the whole grid down. We learned a lot from the 1993 blackout. Also, we have satellites around the Sun that can send us warning at the speed of light, a lot faster than any solar mass ejection would travel. In the worst case— a super-Carrington event—the grid can be shut down and saved.