DALLAS—As they filed silently past the disheveled twentysomething staring intently at his iPhone, each passenger on American Airlines Flight 176 to Miami on Friday reportedly had a different theory as to how the occupant of seat 3A managed to get into first class. “I don’t know, he looks way too young to be a CEO and not cool enough to be in a band—I’m picking up a sort of scion-of-a-wealthy-family vibe, but I could be wrong,” said Annie Tolbert, her theory joining those of 228 other coach passengers who each made their own speculations about the man probably being an early employee of a startup who had cashed out when the company went public, a child of divorce who had amassed a fortune in frequent-flyer miles shuttling back and forth between his parents, or perhaps a close friend of the pilot’s son. “Actually, this flight’s pretty full. Maybe he just got a free upgrade.” Sources elsewhere on the plane claimed the young man didn’t look that excited to be seated in first class, so he must do this all the time and couldn’t possibly have just been bumped up.