In the US the S&P 500 is now down more than 10 per cent from its peak, to sit at a 14-month low.
And as often happens, this is being mirrored by declines in Australia (click to expand).
There's always a lot of commentary after the event, of course, and it's impossible to time these things.
But the very simplest explanation is probably that there's a lot of corporate balance sheet debt around today, which had helped to pump US stocks to very high levels.
And there is also an awful lot of government debt, as well as hidden liabilities such as social security costs.
And now the risk of higher borrowing costs is hitting confidence after years of exuberance, with higher rates to roll over US real estate and then stock markets.
The Federal Reserve is expected to hike rates in December, and traditional indicators such as the yield curve point to twitchiness about a possible US recession.
