People assume that it's artificially high because of people buying bitcoin to get their money out elsewhere, but that doesn't fit because the price isn't unnaturally low on other exchanges. Bitstamp is acting as it usually does, at least it was a few days ago, staying about a 3-5 dollar distance below. It has been volatile though, because of some large whales pushing the price around, blowing lots of stops, and generally shooting the gun too soon over and over.

The reason Bitstamp is usually lower is because it receives less initial deposits of cash, and because Bitfinex allows shorting through them, which allows for selling other people's coins in hope of buying them back lower.

The reason BTC-e is low, is because of the altcoins they have, such as litecoin, which is big right now. This saturates their market. They're about as low as they usually are.

As for the others, I do not know, could be any reason for why they have the price they do.