Ok, one more on the big drop

It will take some time to know precisely what happened, but one thing is clear: This was not the result of a single large player making a single bet.

and

Around two-thirds of all the trades during Thursday's extraordinary “flash crash” that Nasdaq and the NYSE say they are likely to cancel involved exchange-traded funds (ETFs) or exchange-traded notes (ETNs), though the reasons for this aren’t yet clear
---IndexUniverse, 5/7