The lines didn't "break" or "fail", they "were severed".
I want to ask "by whom?"
Interesting wording.
***The lines didn’t “break” or “fail”, they “were severed”.***
Probably means something happened at the plant to cause the motor operated disconnects to the line to open thus “severing” the connections.
The sudden loss of load may have caused the OTHER connections to see a rapid drop in load causing them to open as a protection to protect their end of the grid.
Then there may have been a ground fault in the system causing the protective relays to open.