In early October, top executives of two large independent production companies, Chesapeake Energy Corp. in Oklahoma City and XTO Energy Inc. in Fort Worth, separately sold large blocks of personally owned stock in their companies.Because of intense market fluctuations in recent weeks, some investors have been forced to liquidate holdings in the commodity markets to cover margin calls in equity markets. But these sales of company stock by three corporate executives appear to be isolated incidents, analysts said. “If there were any more CEOs with investments at risk, more stock sales would have happened by now,” said Ray Deacon, senior analyst at Pritchard Capital Partners LLC i...