Don't know how Poloniex is doing it .
From my experience in BTC futures market , people can't manipulate the price on a single platform , because they need to control the price on multiple platform at the same time in order to effect the price greatly . Also , the smaller the exchange , the smaller the weight would be in the calculating factor .
Although I'm not sure that's what happened . You shorted or longed BTS ?
If you longed , then your margin call may happened at whatever fluctuation lower than 100% (5% maybe if it was a 20X leverage) , then it was margin called and forced to sell the BTS at the market with low liquidity which may account for the 50% drop in price chart , which was after the margin call but not before ?