The roller-coaster career of maverick trader Victor Niederhoffer took a sharp downturn after losses mounted and a key investor withdrew money from his firm, demonstrating how the market's recent volatility has shaken even some veterans. Mr. Niederhoffer's hedge-fund firm, Manchester Trading LLC, ran into difficulties a decade after Mr. Niederhoffer lost most of his personal savings when his previous hedge fund collapsed. Last month, Mr. Niederhoffer's largest hedge fund, Matador Fund Ltd., was liquidated after suffering losses of more than 70%, according to people close to the matter. Adding to Mr. Niederhoffer's problems, according to a person close to the...