The reason for short futures lies in the trading system of margin in futures. The so-called margin system refers to the system that investors only need to pay a certain percentage of margin when they open positions without paying funds according to the market value of futures. However, when the market is unfavorable, the amount of loss cannot exceed the actual cost of investors' opening positions, so once the price fluctuation reaches or approaches the margin ratio, the exchange will force investors to close their positions.