This was an unusual day for this year: stocks started selling offright at the open and never recovered. Is that unusual? Yes, for this year. The usual pattern has been to bottom within an hour after the open, or at most by the European close, then rally. That didn't happen today.
Indeed, declines of almost any kind—let alone four days—is unusual. The Dow has had only one other 200-point decline all year. Another unusual feature of today's selloff: even defensive names (Kroger , Coke , Colgate ) got hit.
Another unusual feature of today's trading: volume! The NYSE floor traded 970 million shares; most days this year it has done between 700-800 million. This is largely because volatility picked up; when this happens high-frequency trading computers literally "kick in" and begin trading more.