gox is the biggest exchange and you need a LOT of coin or money to move that market or at least to get people to start panic selling or buying.
the other exchanges are much smaller and you can get the trend going in the opposite direction quite easily.
what would happen if people used relatively less money to move btce, vircuex, bitstamp etc the opposite direction towards 90$. what people on gox notice and change their sentiment upwards?
If the BTC price on one of the smaller exchanges is much higher than on MtGox, people will just move some coin from Gox to the smaller exchange, sell there, move the fiat back to MtGox to rebuy at a lower price. This will push the price down on the smaller exchange and up on Gox. The ratio between these price movements depends on the volume required. With a small volume, Gox prices will barely move while the person doing the arbitrage is pushing the price on the small exchange back down.
With Gox currently being very slow with USD withdrawals, the price on Gox will typically be equal or higher than elsewhere, since people can't withdraw after selling to run arbitrage with smaller exchanges. The other way around, taking BTC out of Gox, selling on other exchanges and putting the USD/EUR back into Gox to buy works fine. Which is why you won't see smaller exchanges with higher prices.
