The lower the exchange rate, the more volume you will see of the asset being traded. (It takes more volume to trade the same value.)
This isn't rocket surgery!
In this case we are dealing with fake Bitcoin IOUs issued by a dysfunctional, incompetent, possibly insolvent company. These IOUs are proving to be practically worthless, as they should be. Although, the fake fiat IOUs are probably just as worthless.
However, both Bitcoin and Fiat at Mt.Gox are fake. So how is it the price of BTC lower relatively?
The explanation for this is that there 5 times more fake Bitcoin than what really exists. Furthermore, Mt.Gox does not dare to fake USD.