Someone put on a bearish position in the S&P 500 just 10 minutes before the market took a dive Thursday, suggesting the market swoon was less a mistake and more the result of some traders exiting a carry trade, hedging, or outright speculating. In any event, the much discussed "fat fingered" trader might not exist.What's clear is that once some computers hit triggers to sell, that selling pressure triggered other computers to sell and within minutes the rout was on. It also happened at a time of day when margin calls would have snowballed. Data from Interactive Brokers shows 48,000...