When Poloniex and Bittrex both close a currency, it's likely there's a security issue with it. At least in the past it was because of double-spend or other wallet issues. They have also been accused of doing this when there's a run on a coin ( mass exodus/sale).
RE: WTH? Both Poloniex AND Bittrex Have Set Their Steem Wallets Offline!? What's The Solution? I Need To Access My Steem Reserve.