NEW YORK (Reuters) - Fallout from the mutual fund probe erupted on Monday as the head of one of the nation's biggest fund companies was forced out and securities regulators found more firms may have engaged in improper trading. U.S. regulators also accused nearly 450 brokerage firms of overcharging investors for mutual fund purchases and said tens of million of dollars of refunds may be necessary. The U.S. Securities and Exchange Commission (news - web sites) told lawmakers at a Washington hearing that 10 percent of top mutual fund companies may have been involved in illegal late trading. Once seen...