NEW YORK (MarketWatch) -- The U.S. stock market's rapid freefall Thursday afternoon was accelerated by program trading, which was triggered after a sharp drop in shares of Procter & Gamble (PG 60.65, -0.10, -0.17%) and at least one other Dow stock, 3M Co. (MMM 83.06, -1.18, -1.40%) , market watchers said.
Shares of Procter & Gamble plunged to $39.37 from around $60. The New York Stock Exchange said each stock has its own circuit breaker level. When these stocks fall below their levels, then they can be traded on any other exchange or platform at any price. When P&G fell below its circuit breaker, a bid came in for the stock at $39.37 from the Nasdaq, the NYSE said.
"You don't see a blue-chip stock like this go down 20 points with no news, said Frank Ingarra, co-portfolio manager at Hennessy Funds, a quantitative firm that deals with program traders. "All of the algorithms kicked in from this errant thing."
