So Coinbase must be collecting lots of bitcoins and depleting their dollar reserves. And they cannot sell those bitcoins now without a substantial loss. This is what MtGOX was doing until a couple of days ago. (Perhaps Coinbese is trying to fill the market niche that MtGOX abandoned? )
Perhaps they believe that the prices will rebound to more than 730 soon?
Is there a general shortage of money in the other exchanges, that might be cured when the banks open today?
What is preventing arbitrage from working (buy cheap coins at other exchanges, sell them to Coinbase for 30%+ profit)? We knew why in the case of MtGOX, but that explanation does not apply to Coinbase, does it?
I assume they gradually raised their quoted prices (importantly both buy and sell prices) until the sell demand started to overtake the buy demand. Combined with a spread of almost $20 between buy and sell prices, they should still be making plenty of money without exposing themselves to very much risk.
There is nothing preventing arbitrage from happening. In fact, it almost certainly is happening right now. It will only last until they wire more money in tomorrow and the price works its way back to the normal market price. It does hint toward significant US buying demand right now, which suggests an impending rise in the morning.