Unfortunately, such events are so distant and heavily mythologized that whether or not there was a inciting event that actually happened or not is pretty much impossible to say.

Folks like to say “every myth has an element of truth to it,” often as an excuse or justification for there having been some sort of event that “got blown out of proportion” so to speak, but that’s not necessarily true. As /u/-Geistzeit says in another reply, dragon-slaying is a common motif.

Let’s take a brief look at one famous dragon-slaying that I’ve researched more thoroughly, and allegedly occurred more recently: The Story of Dieudonne de Gozon and the Dragon of Rhodes.

Briefly, the story goes that during the time the Greek island of Rhodes (Rodos) was controlled by the Knights Hospitaller, the island was being terrorized by a dragon. The knights continuously failed to slay the beast until the Grand Master forbade any further attempts. One knight, Dieudonne de Gozon, ignored this order. He took a leave of absence back to his family’s castle in southern France where he built a model of the dragon, stuffed it with meat, and trained dogs to attack the soft underbelly. He then returned to Rhodes and with the help of his trained dogs promptly slew the dragon and cut off its head. The townsfolk of the City of Rhodes hailed him as a hero and nailed the severed head up over one of the gates of the city. Sir Dieudonne was chastised by the Grand Master for disobeying the order to not harass the beast and briefly stripped of his habit, but was quickly reinstated. When Sir Diuedonne eventually died, his tombstone read “extinctor draconis”—“here lies the dragon slayer”.

There are some elements of truth in this story. Dieudonne de Gozon was a real person. He was Grand Master of the Hospitallers from 1346 to his death in 1353. He was indeed from the south of France; his family’s castle still stands just a few miles south of the city of Millau.

Several historians relate this legend when discussing the history of the Knights of the Order of St. John. Many give an ambivalent “well, maybe something happened. Maybe it was a crocodile or a big snake, or several snakes,” but any such statement is speculation.

Furthermore, there’s been a considerable amount of embellishment to the story over the years. That supposed epitaph “Extinctor Draconis”? Yeah, de Gozon’s tombstone is in the collections of the Cluny museum in Paris. Those words appear nowhere on the stone. The head nailed up above the gate? Some versions of the story say it was conveniently disposed of in the 19th century when the castle was undergoing renovation (ignoring that the Hospitallers themselves extensively rebuilt the fortifications at least twice before they were ousted from the island by the Ottomans in 1522). Another telling goes that the head was removed after an unnamed scientist visited the island sometime between the 18th and 20th centuries and identified the skull as a crocodile. There is no evidence of either scenario happening.

Then there’s the observation that the story itself was essentially unknown on Rhodes until introduced in modern times. Prior to this, the story was peculiar to southern France.

So, what does this mean for the veracity of any claims of an inciting event for famous dragon-slayings in history? Basically, these events are so heavily mythologized and so layered with added fictions, that we ultimately cannot know if there was a real event at the heart of the story or not. To say one way or the other would be pure speculation. Maybe Dieudonne de Gozon did kill some reptile on Rhodes, or maybe the story was made up by someone in southern France. Maybe some animal did terrorize the folks around what’s now Tarascon around 48AD, or maybe the story was pieced together people in southern France sometime in the Middle Ages . The story of St. George killing the dragon was attributed to several other saints before finally being attributed to St. George sometime in the 11th century.

In summary, there’s no way we could know if there was an inciting event short of finding a primary account of the event in question.

These sorts of questions appear here frequently, largely because of a bit of folklore that leads many to believe that "beneath every legend is an element of truth." There is no real basis for this, and most "eureka" moments when someone claims to have found the inspiration behind a folk legend is usually speculation that cannot be proven.

Several years ago, I wrote a response to these sorts of things, codifying it into an appendix for my Introduction to Folklore which I used when teaching the subject. The following excerpt addresses St. George as well as other legends of this sort:

When I see the posts like this asking about whether there were real people or events behind legends, myths, and/or the ancient gods, I respond with several observations. First, the idea that the gods and heroes of legend are based on real people had an early proponent in the Greek, late-fourth-century BCE writer, Euhemerus, giving his name to this approach to myth and legend: Euhemerism. Folklorists generally regard the idea that there was an actual basis for most oral tradition as barking up the wrong tree, because the original “real” event behind a story is usually elusive and searching for that core is a futile exercise. In addition, research into how stories began usually concludes that they emerge in a rather spontaneous way, typically without an actual incident to inspire them.

A few examples: the Classical Greek story of Perseus is an early manifestation of a widespread folktale, catalogued by the twentieth-century folklorists Antti Aarne and Stith Thompson as AT 300, “The Dragon Slayer.” Was there a proto-Perseus who rescued a maiden from some sort of extraordinary threat or perhaps from some sort of human sacrifice? It is hard to answer that question, but it is not hard to imagine how far back in time that proto-incident would have had to occur: AT 300 is spread throughout Eurasia. It was collected from cultures that could have no conceivable literary connection with ancient Greece, and yet the shared assortment of motifs in the numerous variants clearly show some sort of genetic, that is, historically connected relationship. Would we need, therefore, to go back thousands of years before the first recordation of the Perseus story to find this proto-Perseus? It is much easier to understand that the folktale simply diffused and that one of its manifestations was in ancient Greece.

Now, let’s consider another example that has inspired a lot of spilled ink. A simple Google search for the “origins of King Arthur” provides more websites than one could easily read in a week. Was there a proto-Arthur? Perhaps. Maybe there were several. But what does that prove? Every society has remarkable characters, and it may be a natural process for these sorts of individuals to attract all manner of traditional stories that have nothing to do with the original inspiration of the cycle of legends.

So, what do we have with Arthur? Was there a core source (or sources) for this legendary character? Let’s concede for the sake of argument that the answer is yes. Now, did this individual have a great warrior at his side who became ensnared by the leader’s wife in the fashion of Lancelot and Guinevere? That is more problematic since this type of story is also associated with Diarmuid and Grainne in the Irish court of King Finn and with the Cornish stories of Tristan and Isolde in the court of King Mark. One could even argue that it is the story behind Helen of Troy. In fact, it appears that this was a widespread type of story that became associated with various courts of historical legend. We cannot conclude that every great king had a queen who was attracted to one of his warriors and coerced him to take her away. This is simply a story that was attached to cycles involving great courts. In short, the further one goes back to find the “real Arthur,” the less the candidate (or candidates) look like the King Arthur who has been beloved for centuries. The proto Arthurs are not really King Arthur. They may be seeds but they look nothing like the tree that would grow over the centuries. We do not hold an acorn and say “Ah, I have in my hand a mighty oak tree.” It is not yet a tree. It is a seed. And the two look very different even if they are genetically linked.

One more example: there is a widespread legend told by countless families of the ghostly appearance of a loved one in anticipation of news that the individual died. This became a popular tradition in post-Famine Ireland because so many relatives lived in North America or elsewhere. But it is frequently told by all sorts of people internationally. So, we can ask, are there real-life, actual inspirations for this legend? That is, do the spirits of the dead actually come to visit loved ones? Well, how the hell should I know? To paraphrase a famous line from the television show “Star Trek,” “Damn it Jim, I’m a folklorist, not a ghost hunter.” And I have no intention of becoming a ghost hunter. It doesn’t matter what is behind stories so much as it does that people tell these stories. I’m in it for that part of the game; I consider stories as they are told over time, to gain from that material some insight into the past, into culture, and into the human condition. I am a folklorist. And with that, my plate is full.

I'd like to add that while historical chronicles from the Middle Ages sometimes record dragon sightings, these are not (to my knowledge) ever accompanied by contemporary accounts of anyone actually going after the dragons in question. I'll quote part of a previous answer I wrote related to this topic:

There are other cases where we know that a basic folkloric belief was shared among people in Britain while knowing very little about the variations and transmissions of that belief. For example, OP asked about dragons. We know that some people literally believed in dragons - for example, monastic chroniclers in Iona noted the appearance of flying dragons in 735 ('A huge dragon was seen at the end of autumn followed by a great peal of thunder') and 746 ('Dragons were seen in the sky'). Does this mean that the chroniclers who saw these dragons had the same beliefs about them that an Englishman listening to a recounting of Beowulf would have? Did all people who lived in places where references to dragons were produced actually believe in them as real creatures they could meet? Or did some of them think they were only the sort of thing that happened to great saints like Michael the Archangel, or long-dead heroes? We really don't know. (If you'd like to read more about cross-cultural comparisons between the Beowulf dragon and others, check out Christine Rauer's Beowulf and the Dragon: Parallels and Analogues.)

So while we do have monastic chroniclers faithfully recording that dragons were spotted, these are described in more meteorological terms than adventurous ones. The chroniclers wrote a lot in Ireland and Britain about soldiers killing other humans, and they wrote about dragons being spotted in the sky, so you'd think that if they ever heard of a soldier killing a dragon they might have recorded that too.