There's also a rebalancing of the S & P 500 at the close.
This doesn't usually cause a lot of mayhem, but there have been an unusually large number of options contracts that were still open Thursday.
S & P 500 option open interest (puts+calls) (contracts, target price, followed by contracts) 4,000: 300,000 3,950: 150,000 3,900: 125,000 3,850: 80,000 Source: Interactive Brokers This is a lot of open interest.
"Once it was clear 4,000 [on the S & P 500] was not holding, we sold off quickly and abruptly," he said.
The index options prices are set by the opening prices on the S & P 500 on Friday, so expect a huge rush of volume at 9:30 a.m.