The day bitcoin futures dropped was the first day that traders were allowed to bet against it

Cboe bitcoin futures fell Wednesday in their worst day since the launch, just as Interactive Brokers began allowing clients to short, or bet against, the futures.
The brokerage had said Monday it had not allowed shorting "due to the extreme volatility of cryptocurrencies."
But in a Wednesday release its CEO said it would introduce short sales since the January futures contract was trading at a much higher price than where bitcoin itself was trading.