We don't know a lot about the technical problem that shut the NASDAQ stock exchange down for three hours today.
As with the recent Amazon or Google outages, the most likely scenario is that some bug buried in zillions of lines of code caused some cascading failure. But hell, maybe it was something weird (like squirrels). Understanding the hows and whys of these systems isn't easy from the outside.
During 2010, when the markets experienced a "flash crash," a whole SEC investigation dug in and found a firm's algorithm had made a simple mistake. And that, for whatever set of contingent reasons that day, set off a panic. Why that day? Why that magnitude of effect? It's very hard to know.
What we can say is that when things work in new ways, they break in new ways. And that's a theme that's worth obsessing over.