It’s based on similar principle to the 2003 northeast rolling black out. Power systems that are connected get to a critical state and then it’s impossible to isolate the fault in the system. Much the same as a virus that spreads much faster in a globally connected world. check out this page in wikipedia
What happened on Wall Street was similar. As a lot of the trades are electronic and programmed and the exchanges are all connected a glitch in one part of the system makes the trades go else where. The problem can happen without electronics and the connection but the more connected it is the more the chance of catastrophic failures.