"We know what happened wasn't due to any major news event," Marketplace's Stephen Beard points out. But it is hard to find alternative reasons for the plunge as well. One theory, he says, is so-called "fat-finger syndrome" -- otherwise known as human error.
"Error is a possibility," says Nick McGregor of brokers Redmayne-Bentley, "but it would have to be a very significant error to have seen the volume which we saw in this contract -- nearly double in the course of a couple of hours."
Some analysts in the U.S. say it must have been high-frequency trading, adds Beard.