OK, well maybe I should qualify my admidedly smug post above...
What I think has happened is that there ha sbeen a fat finger / computer failure, and that has triggered a boat load of black boxes into action; with the market on tenterhooks concerning Greece (and Spain, UK elections etc) the sell off caused plain old panic - look across other asset classes.
The FF looks like it was in Proctor and Gamble (chart attached). Two thing to note about P&G
1. It's a consumer stock, just about every Joe Soap has some tucked away in their bottom drawer
2i. It's occupies a 4.32% weight in the DOW
2ii. Now P&G dropped approximately... (consults calculator)... 36% - which will have made the concimant effect on the Dow about 1.5%; Now, P&G is also in the Spoos (SP500) - but at a weighting of 1.71% (ergo the change in the SP500 will have been only 0.6%)
Now, I am no quant (at all, in the slightest), but I reckon the following happened;
** the news about Fat Finger has just hit the wires, so I put it above - still havent finished this post!**
1. The FF in P&G triggered a shitload of stops in retail ETF's (retail as in your gran has it in her ISA, Pension funds etc...)
2. The black boxes were arbing the Dow vs. S&P [and others] - which meant selling a shit loas of S&P futures (apparently 2000% of average volume, circa $900B)
3. Once the selling in the S&P started, panic quickly spread on Greece fears and the sell off began.