Risk parity funds. Volatility-targeting programs. Statistical arbitrage. Sometimes the U.S. stock market seems like a giant science project, one that can quickly turn hazardous for its human inhabitants.......While it’s impossible to say for sure what was at work when the Dow Jones Industrial Average fell as much as 1,597 points, the worst part of the downdraft felt to many like the machines run amok. For 15 harrowing minutes just after 3 p.m. in New York a deluge of sell orders came so fast that it seemed like nothing breathing could’ve been responsible.