Richard Bove, an analyst at Punk Ziegel,  suggests technical or program trading may be responsible for the wild midafternoon swings.

Others speculate that a large investor -- perhaps even a foreign buyer such as China -- may be targeting the U.S. markets.

In a report, Bove cites an article

 from the Financial Times saying

 the Chinese have the equivalent of some $4.8 trillion in savings

 to put to work.

That's making them more apt to make riskier investments in foreign markets.

"I certainly have no idea where this is coming from," Bove said.

But "it takes a huge amount of money

to move the equity markets

 that sharply in so short

 a period of time."