Why are the USD exchanges not following China? ^^
China exchange has a coin shortage.
What happens if they run out?
I suppose if there were all buyers and no registered sellers on an exchange and the buyers placed all their coins in cold storage the exchange could run out of coins the same way bitstamp seems to have run short of fiat.
Bitstamp short of fiat because the banks closing at weekend. But short of BTC only means no sellers (more precisely no one put ask orders) since BTC transactions are 24/7.
No ask orders does not mean there are no selling pressure. Sellers could be hesitating at predicting the trend. They may keep watching the market and put the mouse over the 'market sell' botton now. Some sellers may also set the selling threshold on their trading bots.
Therefore, crash may happens anytime. Just needs some one pull the trigger.