It's much quicker to deposit BTC in Mt.Gox than it is to deposit USD, especially for people unable to use Dwolla. Since it's easier to sell than buy, if the market price is falling the effect maybe amplified as it takes longer for buyers to get money in to counteract the fall. Also people are more comfortable leaving BTC in their account rather than USD which skews the order book. If it was quicker to get funds into Mt.gox accounts then it might help smooth out market fluctuations and weekend trading.
If you have BTC in your Mt. Gox account you can use it as collateral for a temporary loan, equivalent to half the value of your holding, which you can use to buy more BTC. When a loan is outstanding no BTC can be withdrawn. If the loan is not repaid within a specified time frame (72hrs?) then entire BTC from your account is confiscated.
I have 600 BTC in my Mt.Gox account and market price has dropped to $10. I can use my 600 BTC as collateral to buy 300 BTC more @ $10. I then either have 72 hours to deposit $3,000 USD or sell a proportion of my BTC or I loose the entire 600 BTC.