the problem is Poloniex. They have SBD in maintenance for I don't know how long, and people are just panic selling.
Take a look here: https://coinmarketcap.com/currencies/steem-dollars/#markets
Bittrex: $0.94/SBD (I've been monitoring the price on bittrex, as I'm using this exchange, and SBD is sitting around $1)
Poloniex: $0.78/SBD (people panic-sell their SBD, as they want to cash-out)
I don't think maintenance should create panic sells although a lot of people don't know what exactly means and start to sell. Usually the coin come back to normal trades after some time .
The only thing that could cause panic sells is a coin to be delisted in a crypto-exhange platform (happened to poloniex in end of April which made a delist of i think 18 coins) .
Actually, from what I gather around, poloniex has maintenance mode on SBD for many days (don't know for how long), and people are selling to (hopefully) get out of poloniex.