The triggering event is a high altitude nuclear explosion designed for a maximum EMP pulse. I think China and Russia (and a few of our close allies) are the only actors who have this capability and I think there is an astronomically low chance of them doing this.

Still, spending money hardening our infrastructure would improve general reliability, help prevent deaths do to storm outages, etc. Good thing to do.

This is one of the big worries about Iran's nuclear program. 3 IRBMs, which they have, lofting 3 warheads (you need that many to get fairly complete coverage of the CONUS), they might be able to pull it off.

But it would be a terrible gamble for them, and most any scenario besides total failure results in the country being turned into the proverbial "fused glass parking lot".