I thought that they instituted changes to the computer-trading software that were supposed to shut the things down, to prevent cascading selloffs going exponential. It seems not. They claim that it was because of a glitch when someone noticed that computers weren't "properly handling" a massive selloff, so they switched to a backup system to work around it.
I have an idea of what really happened, and it's something called "human nature." I think the computers shut down trading, as they were supposed to, when a certain limit was reached. And the humans involved thought the shutdown meant something was wrong, rather than that the systems were doing what they were supposed to do. So they essentially wired around the safeguard. Whoops, we just dropped 200 points in about a second and a half. Whaaaa??? The trouble with idiot-proofing the systems is that idiots are so clever.