The sudden downwardmove caused stop orders for many stocks to be executed, leaving some shareholders with unanticipated losses or realized capital gains.
In more blunt terms, a downwardmove by domestic large-cap stocks now has an increased chance of dragging down emerging market stocks, REITs and commodities with it.
These folks, for instance, divine that something called a "head-and-shoulders top" formation--a market high above two high points on either side--means that stocks are poised for a downwardmove.