A trader essentially takes a loss on both the collateral and the futures contract. Thus, margin requirements increase at a faster rate with price declines, and longs get liquidated relatively quickly. That, in turn, puts further downward pressure on the market, leading to a deeper slide, also known as leverage washouts, like the one seen in May. Long liquidation refers to forced closure (forced selling) of bullish positions due to margin shortage.
www.coindesk.com