For several days this week, the last hour of trading has seen a pullback even if stocks were higher throughout the day.
Art Cashin, director of floor operations at UBS Financial Services, told CNBC that the last-hour pullback is based on worries about Europe.
"The market has been almost like flipping a coin in that final hour," he said.
"Much of the focus remains on Europe," Cashin said.
"So as you get into that final hour lots of people say 'you know what, I don't know if I want the overnight risk,' and you begin to see some of that trim back."
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No immediate information was available for Cashin or his firm.