I've said before that exchanges like to manipulate value of currencies like Steem because the total amount of coins traded in exchange is more than the exchange has in the wallet used for withdrawals. I think Bittrex is the only exchange that has more Steem in the withdraw wallet than traded on the exchange.

When Steem is close to go moon, exchanges disable wallets so people can't transfer more coins to the exchange and that will cause avalanche effect, especially if the exchange has both Steem and Steem Dollar.