Symantec shares are tumbling on the surprise announcement Steve Bennett has been fired as CEO and president. The termination marks the third CEO change in five years as the security software company struggles to implement a turnaround. The stock is down 23% year to date as declining PC sales dampen demand for its portfolio of antivirus software. Over the past months, management has also issued soft guidance. For its fourth quarter ending March, the company expects revenue to fall as much as 7%.