If I can make a wild guess, people would rather hold usd than btc so at least they can make legal action against gox when things go belly up.

so people sell btc to usd at all cost.

it is a good opportunity to double your btc (sold at 600 bought back at 300) and have a lot more btc on your account what you will never be able to have, niiiiiceeeeee!  (of course it was irony)