Silk Road provides continuing volume at any prize. It wouldn't matter if it was 1000 BTC/USD or 10 BTC/USD - they need the same USD sized volume to turn over to finalize deals.
There is a simple explanation for that. Silk Road is the real business. They have no heroic, ideological investment in Bitcoin. For them it is just a very convenient tool. They don't care if BTC is $10 or $100. They want volume and commission on trades and to keep customers happy. For more volume and more commissions.
I'm guessing they started with a good amount of BTCs and cash available, to maintain the liquidity initially.
MtGox, OTOH, may be in the market cornering business.
In the olden days, there were guys who had enough money (or could get enough financing) to, for example, buy entire corn supply of the country, before anybody noticed it.
By the time everybody else realized that it is a single guy who owns all the corn, it was too late. The guy, such and such, was then publicly known to have cornered the corn market. Everybody had to go to him to buy the corn and he could dictate the price. These guys became rich and famous, but some did, in the end, lose their fortunes.
It is possible that MtGox has agreements with a couple of groups, controlling large amount of BTCs, to corner the market together.
