Most grid systems have modern monitoring gear, but the working end (The Switches) are mechanical/electric vacuum breakers and transfer switches. The get a signal or lose a signal and they operate. They have built in delay timers and re-connects that are simple and uncomplicated for reliability and usually are 99% reliable. (that is why there is built in redundant for the 1%)
The modern stuff is just a shiny version of the old. Something weird happened and I cannot figure what. My guess is that a big power plant dropped off line to soon. A technician could have mis adjusted a trip or a timer failed perhaps.
But the area where the fault was that started the cascade should have dropped from the grid prior to the power plant having to disconnect.
So when they report that the nuke plants did an auto shut down .. it wasn't really .. a tech would have to shut down it?