In days of yore this was considered quite ominous, ebbing strength a portent of a bull market's expiry.
Now it probably means someone has tripped over the power cord at one of the big algo futures traders (think drunk partner at RSJ),
From Bespoke Investment Group:
As shown in the chart below, for the fourth time in as many days, the S&P 500 closed today down more than 1% from its intraday high. Just a week after one of the broadest equity market rallies in decades, sentiment has shifted on a dime, and investors are taking the term 'sell into rallies' to a new level....MORE