Why did Billie Joe McAllister jump off the Tallahatchie Bridge?  It’s almost as big a mystery as why Bobbie Gentry vanished from view after 1981.  Happily, the enigmatic singer-songwriter left behind an enduring and diverse body of work which is now being celebrated on a comprehensive box set from Universal’s U.K. arm.  On September 21, the 8-CD collection The Girl from Chickasaw County: The Complete Capitol Masters will collect all seven of Gentry’s 1967-1971 Capitol Records studio albums plus more than 75 previously unreleased recordings.  This treasure trove of lost material includes…