Cliches are boring. They are as boring as watching paint dry. Everybody knows this; everybody's heard them too many times for them to truly mean anything any more. They are the tool of the lazy writer, of the panicking commentator, of the tired thinker. They're rubbish.
They're also, sometimes, really quite weird. Take "torrid". Non-scoring strikers are often having a "torrid" time of things. Future generations will doubtless assume this has something to do with Fernando Torres, yet the word actually means exposed to a searing, parching, burning heat. A torrid time, then, is one spent floundering across a desert, desperate for water, stumbling in the sands. Lost, miles from home, without even a friendly camel to sit on. Seeing -- oh, bliss! -- an oasis, then realising -- oh, woe! -- it's a mirage. That last is probably missing an open goal, or something. Anyway, you take the point: every cliche, however worn out it is, comes from somewhere.
Now, take "relegation dogfight". Back in the day, a dogfight was just what it sounds like: "A battle between two dogs" according to Webster's 1828 Dictionary of American English. Dogs have been fighting for the entertainment of crowds since at least ancient Rome, first against larger animals in packs ("baiting"), then in the 19th century, when bears and bulls became more expensive, against other dogs. As you'd expect from a fight between two well-toothed lumps of systematically abused muscle, they are unpleasant and brutal things, and can last over an hour. Should either dog show a submissive gesture, they are returned to their marks and released again. The losing dog is, if not killed in the ring, frequently injured beyond further usefulness. If they're lucky, that means a bullet.
But that was then, and football wasn't even football. Interestingly, the early uses of "dogfight" in relation to football had nothing to do with relegation at all. In 1952 The Times -- which has the double advantage of being both the British newspaper of record and the only archive to which your correspondent has immediate access -- predicted that "There should also be something of a dogfight at the head of the Third Division (South)", and then in 1964 noted that "the dogfight at the head of the Fourth Division continues".
The "relegation dogfight" doesn't arrive in The Times until the climax of the 1980-81 season. On April 6, Norwich City got lucky, as "three of the other four sides involved in the relegation dogfight lost." Then, on April 27, a headline: "Defeat bring Sunderland into relegation dogfight". Eventually, and contrary to the traditional Warner Bros. narrative, the Canaries lost out to the Black Cats and were relegated with 33 points, two behind Sunderland, Wolverhampton Wanderers, and Brighton & Hove Albion.
By this point, the definition of "dogfight" had changed. Hunting aside, by the 20th century animal-centred bloodsports were mostly out of fashion and banned by law in most of the English-speaking world. War, on the other hand, was very much in, and while the majority of the First World War was a muddy, depressing exercise in imperial hubris and mechanised slaughter, dogfights came out the other side with their reputation enhanced. This is because the dogs had been replaced by planes, and planes are cool.
Presumably, actually being in a dogfight is a faintly terrifying experience. Particularly in the early days of planes-with-guns, when ammunition was scarce, technology was new, and there was a very real chance that a pilot might end up shooting off his own propeller. Despite this, from Manfred von Richthofen through his arch-enemy Snoopy and on to Maverick, the mythology around flying aces and their swooping, diving, duels in the sky has invested the notion of the "dogfight" with an unshakeable glamour, and the sense that here is a feature of war that can perhaps be viewed as unambiguously fun. In the abstract, anyway.
(Aside: according to Wikipedia, the very first dogfights were just pilots with revolvers taking potshots at one another. And before that, they just used to wave their fists. Which is, let's face it, adorable.)
So, which of these dogfights is the relegation dogfight? Is it the brutal, bloody, cruel spectacle of two poor maltreated animals forced to kill or be killed? Or is the daring, thrilling spectacle of two noble combatants who ... well, are forced to kill or be killed. Or crash. Or just fly away. But have volunteered for all of that, usually.
(The word can also mean just a fierce scrap. But then that presumably came from one of these two origins, or is at least used to evoke one of these two origins, and so that just nudges the question up a level and doesn't really get us anywhere. So quit your pedantry.)
Perhaps it's both. After all, a relegation dogfight is a completely different thing to different people. To anybody lucky enough not to be particularly involved with any of the protagonists, it's a highly entertaining business. Like the aviators, it's full of exciting feints and swoops, as the teams charge around one another. Here's Crystal Palace, hiding in clouds! There's Sunderland, diving down from the sun! Metaphorically! It's great fun to watch.
Whereas when you're stuck in one -- when you're the dog, or when you give a flying one about the dog -- it's a dreadful business. This is not to assert that getting relegated is as serious or as damaging as having your neck savaged until you pass out before being summarily executed; it's not a literal dogfight, Jeff. It's to say that the power of the metaphor (or cliche) resides in its ability to evoke two contrasting-yet-related kinds of conflict. It is, or it was, quite a delicate piece of description. For a term that's usually just a lazy default, it's doing an awful lot of work.