Southwest Airlines Co., proud owner of a storied corporate culture that values fun, hard work and a maverick mindset, is contending with an unaccustomed challenge: hostile public disputes with its labor unions. Employees are showing up for picketing like never before and waving anti-Southwest placards. Unions representing more than two-thirds of the workforce called on CEO Gary Kelly to step down this month, and the carrier’s head of labor relations announced his retirement late Tuesday.