I'm sure many stops/margin calls have been hit, exacebating the...

I'm sure many stops/margin calls have been hit, exacebating the drops - but the biggest thing is perhaps just the appreciation of the USD, USD index from 73.5 to 79 in two weeks.

I don't think it's possible for the USD to appreciate significantly from here (index peaked at 90 in recent years). The US economy cannot be competitive with a strong USD, one of the biggest issues facing it right now. Also with their heavy debt load, the strong the USD, the harder it becomes to repay that debt. A strong USD will destroy chances of a timely recovery, so I guess the FED will go to extreme lengths to 'fix' this - I think the threat of QE has more impact than the often cited velocity of money.