By shorting a stock (look that up if you need to) you run the risk of losing thousands of percent, especially if news breaks after trading closes. This investor was short and the stock shot up after market closed. Since he borrowed shares at a very low price, he had to buy them back and return them at a much higher price.
he shorted against the pharma boy who went to jail. basically, shorted KBIO and one night, they announced pharma boy (hedge fund dickboy) bought KBIO and next morning the price jumped like 20x of it's closed price. Brokerage called margin and he had negative 106k balance