Speculation of course, but it sounds like the whole system was stretched to breaking point. One power station goes off line due to a lightning induced fire and the whole grid comes down like a pack of cards.
The quote of the day has to come from the BBC:
http://news.bbc.co.uk/2/hi/americas/3152451.stm
"I had to get out of the house," she said. "It was so dark, and everything was off, and I was scared. No air conditioner, the phones - they need electricity. And it's so, so hot."