Rethinking a little, in terms of potential causes. Ranked in the order of what I think is their likelihood, although it's possible that a combination of any of the causes below could cause a blackout:

* Solar storm of a sufficient magnitude
* Severe weather, in the context of weather events becoming more frequent and more severe because of climate change
https://assets.climatecentral.org/pdfs/PowerOutages.pdf (See esp the plot on the summary page.)
* Other natural disaster (e.g., earthquake)
* Strategic squirrel or other wildlife activity: https://blog.nature.org/science/2019/10/29/fear-the-squirrel-how-wildlife-causes-major-power-outages/
* Human error, as in 1965
* Large-scale physical problem on or near the grid (e.g., power plant failure, explosion at a nearby facility, etc.) that affects the grid too critically and rapidly
* Cyberattack and/or physical terrorism by a non-state actor (non-cyber physical damage could complement a cyberattack)
* War (e.g., EMP, cyberattack and/or physical destruction) by a state actor; such a large-scale act by a state actor would be an act of war
* Aging grid with a lot of deferred maintenance/updating
* Increasing demands on grid

The forecast period is only 8 months, so I think I'll keep my forecast at 2%. That's pretty high. It's probably too high.