What explains the sudden turmoil?
Perhaps investors had been used to good news for so long that they had become complacent.
In a recent survey investors reported their highest exposure to equities in two years and their lowest holdings of cash in five.
Another sign of potential complacency was the unwillingness of investors to pay for insurance against a market decline, something that showed up in the volatilitv or Vix index.
Funds that bet on the continuation of low volatility lost heavily.
The wobble may also reflect a decision by investors to rethink the economic and financial outlook.
Ever since 2009 central banks have been highly supportive of financial markets through low interest rates and quantitativeeasing (bond purchases with newly created money).
There was much talk of an era of "secularstagnation", in which growth, inflation and interest rates would stay permanently low.
But the Federal Reserve and the Bank of England are now pushing up interest rates,and the European Central Bank is cutting its bond purchases.
Future central-bank policy seems much less certain.
A pickup in global economic growth may naturally lead to fears of higher inflation.
The World Bank warned last month that financial markets could be vulnerable on this front.