My roommate, at the time, was working as an analyst at a large bank. He claimed it was the “fat-finger theory” caused by a trader at his bank. Said some head trader meant to sell (as an example, I can’t remember the specifics he used) $100 million of a security, which is not a small trade and requires a few verification steps to proceed (entering your password multiple times). The trader hit the wrong button and the trade was made to sell $100 billion instead, which requires the same kind of multi-step verification but with these guys moving 1000 mph the million to billion swap was overlooked.