From my vantage point, just about every ta practitioner and his/her brother has been watching that descending channel on s&p500, and expecting it to bounce back down. Which, implies to me that a ton of shorts just hopped on at the diagonal resistance line.
Upshot is that the line was breached -- period.
Meaning the watchers (with the real xray goggles) are quietly embarking on a fade the shorts spree. Keep in mind there is no panic to sell until the next major bomb shell gets uncovered. When others see low volume days like today, I envision the watchers trolling the waters, sipping on java (plumes of smoke dancing in the cold air), with their fancy fishing poles, nets, and transistorized fish-finders, looking to fade the biggest school of fish they can find (while simultaneously avoiding said white whales, killer whales, and/or large predatory blood sniffing sharks).
One entities descending channel is another's breakout flag pattern.