The price of Bitcoin (BTC) momentarily plunged 91% to $680 on the Binance BTC/USDS market, and just as fast as it happened the price of Bitcoin (BTC) returned to normal. This is a prime example of a flash crash and was likely due to an inexperienced trader placing a large sell order with a market order instead of a limit order.
Essentially, a limit order is when a trader offers to buy/sell Bitcoin (BTC) at a certain price, and other traders need to fill this order. A market order is when a trader will buy/sell Bitcoin (BTC) at any available price. If a trader uses a market order for a large sell order, and the order book has relatively low liquidity, then the sell order will eat up all of the buy orders and crash down to the bottom of the order book. This is probably what happened in this case.
Indeed, StableUSD (USDS) is a fairly obscure stablecoin with a circulating supply of only $3 million, and the BTC/USDS trading pair on Binance only has $30,000-$40,000 of liquidity. Data from Binance shows that the sell order which caused this flash crash was for 8 Bitcoins (BTC) worth approximately $60,000, so the sell order quickly crashed downwards through the entire order book.
The trader who did this certainly lost a lot of money, since they sold Bitcoins (BTC) for ridiculously low prices due to accidentally doing a market order. On the other hand, some Bitcoin (BTC) buyers who had previously placed ridiculously low buy orders pretty much won the lottery.
The moral of this story is to always use a limit order instead of a market order, especially when dealing with large amounts of money.