It's hard to clarify exactly what a "duck" is because there's a lot of things behind closed doors that go on. For example, Roberto Duran got stripped of his WBA belt at 154 for not facing his mandatory in Mike McCallum. Instead, he fought the more established, WBC champ in Thomas Hearns for more money. A duck? I think not. Duran was supposed to fight Carlos Ortiz in a non-title bout at MSG, but his management (Carlos Eleta) told him no and pulled out, saying Duran was sick. Eleta also didn't want Duran to have a rematch with Ken Buchanan, or fight Antonio Cervantes at 140. Are these considered "ducks"? Buchanan later said something like "Duran would fight anyone but his management made the decisions". Or what about Floyd Mayweather and Kostya Tszyu? Is Mayweather re-signing with HBO, the more powerful and lucrative network, considered a "duck"? Exclusive HBO and Showtime fighters don't fight each other unless it's something like Lennox Lewis-Mike Tyson which generates $100 million in PPV revenue. People always talk about Ray Leonard "ducking" Aaron Pryor because they watched Legendary Nights. Pryor "called out" Leonard at a press conference, but all the big names are going to get challenged. Fighters want to fight the big names. Does a welterweight champ have an obligation to fight a champ in a lower weight class, when the other champ hasn't done anything at welter? What exactly is a fair definition/explanation of a "duck"?
Not always an easy term to define. As you have already pointed out, fights often go unmade for a myriad of reasons, and usually it has little or nothing to do with having cold feet. One example of a blatant duck that comes to mind, is Riddick Bowe's refusal to fight mandatory challenger Lennox Lewis in 1993. Bowe was given a title shot at Evander Holyfield with the clear understanding that he would have to face Lewis in his next fight. At the post fight conference, he slandered and publicly belittled Lewis, then weeks later, he and Rock Newman disgraced the heavyweight title by disgarding the WBC segment in the trash, and said they'd never give him a shot. It wasn't like they past up Lewis for another worthy challenger with great monetary value either. Bowe's next two defenses consisted of Michael Dokes and Jesse Ferguson. Yuck!!! I think Rock Newman had recurring nightmares of Bowe losing to Lewis in the Olympics, and also of Lewis destroying Razor Ruddock in a mere two rounds.
Define stupid posters on ESB: those who don't know the difference between a "definition" and an "example". To duck another boxer is to go out of your way to avoid fighting them, at considerable cost to yourself and because of the dangers they possess. Contrast this with just "avoiding" an opponent, ie. when they present very little danger but also present very little gain. Tony Thompson has been avoided a lot, but he has not been ducked. Because of the nature of titles and sanctioning bodies, ducking can STRICTLY only be said to have taken place when someone has had a mandatory challenger and has not faced them. In a looser sense, it can be used to describe when a boxer has a very obvious opponent for a fight that the public demands. So, strictly speaking, ducking is very rare in boxing, but it's all too common in a looser sense and boxing is inevitably rife with boxers avoiding other boxers because they're awkward, unpopular, smell bad etc.
Riddick Bowe throwing the "once" most prestigious titles in all of sports in the trash can to avoid fighting Lennox Lewis.