That account is the central account for all deposits and withdrawals of STEEM and SBD into their exchange. Instead of having individual STEEM/SBD wallets, the exchange creates their own version of accounting and internal wallet. The memo attached to the transfer allows the exchange keep track of who has what. When they disabled the wallets, that STEEM and SBD on the Poloniex account is the result of many people's deposits into the exchange.