Quote from heech:
Uh, that represents approximately 3% of average daily volume in ES trades. (And on that particular day, probably far less than 3%.) If that's an "explanation" for the flash crash...
Yes, that's the explanation. The key is not to think of 75,000 contracts as percent of daily volume, but as percent of the total liquidity available during a particular period of time, such as the period of 20 minutes or so during which these contracts have been sold. See the details here: http://sec.gov/news/studies/2010/marketevents-report.pdf
Notable part of the report: "By 2:45:28 there were less than 1,050 contracts of buy-side resting orders in the E-Mini, representing less than 1% of buy-side market depth observed at the beginning of the day".
