Probably the grid frequency started to go significantly below 60Hz. To much load on the system for the power available. Most generators will trip on low frequency. I’ve seen charts tracking system frequency before. When Comanche peak in Texas would trip the system frequency would get as low as 58.5, half way to tripping my customers generators.
It could be a casdade of power plants tripping off-line like the Great Blackout of 2003.
That was caused by one power plant unexpectedly tripped off-line when a high tension line contacted a tree. Then the failure of interconnection breakers to isolate the fault caused other plants to trip off in series.
In the end the blackout extended from New York to Michigan and in to Canada.