Goldman Sachs on today's market action:

"Recent price action is reminiscent of the 2013 Taper tantrum, when the dollar rallied sharply against emerging market FX but weakened versus the G10. Back then, the sell-off originated from then Chairman Bernanke's Congressional testimony in May 2013, where he flagged the possibility of tapering asset purchases, which set in motion a bear steepening in the US yield curve and resulting pressure on EM FX."

USDJPY during the taper tantrum in 2013

Goldman Sachs analysts blamed the recent moves on oil and worries about China.

Ultimately, I don't think we will ever get a full accounting for why markets crashed like they did over the past few days. Certainly there was an element of confusion about the Fed, unease about the US economy and skittishness about the inability of stocks to climb higher.

As the expression goes -- no one knows which pebble caused the landslide.