There are plenty of Black Swans to trip up markets and bulls. Some are known (eurozone, Iran, China economic contraction, a purge in China, North Korea, Syria's non cease fire fighting, oil, currency wars and so forth) and are hovering over investors. Bulls would like to see them as nonthreatening, but then they come back to haunt them like tar balls bobbing to the surface in the Gulf.
Bulls might argue that a correction was due after outsized first quarter gains. They appear to be right for now unless current selling builds on current declines. One thing is clear; equities are short-term oversold so a bounce could occur at any time. Nevertheless stocks were sold hard most of the day with few leading the way lower as selling was widespread.