Why did it happen? Did people at the time even notice? Were there crisis points where definitive change occurred?
All these questions are answered in chapter 11 of Nicholas Ostler's excellent "Ad Infinitum".
"After the collapse of the Empire in the west, Latin began to split into what we now see not as a language but a language family, the vernacular languages of western Europe, collectively known as Romance. ...To restate it: after Alcuin's pronunciation reforms, people suddenly realised that Latin was a different language to those vernaculars which everyone in their diverse ways were actually speaking. It must have been quite a big deal.
"The middle and latter part of the first millennium AD was a period of social "hamletization," when horizons became narrower for many people, and the chances of wide-scale activity, e.g., travel, correspondence, or trade, were highly restricted outside the topmost elites.
"Without an overarching government, movement beyond the local market town became too costly and too dangerous to be undertaken without an exceptional reason. The resulting fragmentation of Latin, hitherto a highly unitary language across its wide range, is the best example we how a former imperial language can split when the political conditions of unity and mutual contact are no longer maintained.
"Latin was transformed on the lips of its speakers into a profusion of different dialects that were one day to become recognized as languages in their own right. The intrinsic changeability of language, the code passed on not quite perfectly from generation to generation, began to assert itself, and the speech of the different communities went off in separate directions.
"The story of how this happened is fascinating in itself, although the changes took place largely unconsciously. To tell it or understand it requires a certain tough-minded determination to see Latin not (as contemporaries did) as rule-governed text on the page, but as a vast set of spoken words, each taking its part in a system, the mental grammar, that made the language make sense.
"As local accents changed the pronunciation of certain sounds, various words' grammatical relations to other words became less obvious, or even quite impenetrable. New generations of language learners made sense of how the language worked in slightly new ways. The changes rippled throughout the system, causing new systems to form, which became the grammars of the new, Romance languages.
"First and foremost, then, Romance is the name for any more or less distorted form of Latin, as the language gradually evolved and split apart in the latter first millennium.
"Over the long centuries in which the new kingdoms established themselves, the stories of Rome's continent-wide imperium and single invincible army came to seem like legends. Societies became more strictly hierarchical, with most people bound into the feudal network of personal relationships, each man (and woman) recognizing his superior lord, but few outside the Church active in that wider world that had once been ruled through Latin.
"Within three hundred years from those fateful crossings of the Rhine, the people of France, Italy, and Iberia began to find it difficult to understand one another when they did meet. Ordinary speech, wherever it was spoken, was more and more called romanica rather than Latina lingua.
"Latin, as a single written language, was still taught in classes of grammatica and increasingly took its name from that. Meanwhile the inhabitants of Rome's old domains increasingly spoke a multitude of dialects, each called an idioma: this, when it could be recognized, arbitrarily marked out the origin of speakers. But the differences between them seemed to have no meaning.
"Nevertheless, there remained the constraint of the need to communicate with the neighbours; in practice the result was not so much a set of distinct idiomata as a dialect continuum, which varied gradually across the whole field of Romance speech. Picking out particular local varieties within this continuum as "national languages" came much later. ...
"At the end of the eighth century, the Frankish kingdom that ruled northern France had become mightier than any other. Under Charlemagne (768-814) it united all of France, western Germany, and northern Italy, with an enclave south of the Pyrenees, and began to act in concert with the papacy in Rome in a way that recalled the glorious old alliance of Church and Empire in the century after Constantine.
"This political revival had an immediate cultural manifestation, what is to-day called the Carolingian Renaissance, when Charlemagne called scholars to receive his patronage at his court in Aachen. They came, first from Italy, but later and more notably from monasteries in England and Germany, and in 781 Alcuin, head of the cathedral school at York, was appointed the director of Charlemagne's Palace School.
"Alcuin was above all a teacher and a regimenter. He presided over the establishment of new standards for the spelling and pronunciation of Latin, an attempt to return it to its classical roots, seen as the source of its fundamental value in education, thought, and culture.
"Alcuin enjoined a new, universal style of pronunciation for Latin, deliberately reconstructed to be close to its original sound. Rather than allow each local community to pronounce its Latin as came naturally, he proposed that all should follow a single norm. In his own words:
Me legat antiquas vult qui proferre loquelas;"This would perhaps give scholars closer access to the true sound of Latin poetry and rhetoric; importantly, it would certainly make it easier for them to communicate orally in Latin, wherever in Europe they might hail from.
Me qui non sequitur vult sine lege loqui.
Let him read me who wishes to carry on the ancient modes of speech;
He who does not follow me wishes to speak without law.
"As a reform, it did not in itself tend towards vernacular literacy: indeed, quite the reverse, for the immediate effect of the new pronunciation was to make priests reading out their sermons or their church offices more or less incomprehensible to their illiterate parishioners.
"In the favourite - somewhat extreme - example, the word viridiarium, 'orchard', could no longer be pronounced in northern France as vair-jair, by then its natural rendering in the local variety of Romance.
"With each priest following his home pronunciation, it was possible - at least in Romance-speaking countries - for the Latin text to have been read pretty much in line with the local language, hence understood by those who could not read. The newly antiquated, universal Latin, by contrast, was a foreign language everywhere, accessible only to those who had studied it.
"So quite soon after Alcuin's reforms, rulings were needed to guarantee that Church services would still make sense. At the Council of Tours in central France in 813, as at the Council of Mainz in Germany in 847, an explicit exception was made, to guarantee the continued understanding of the countryfolk: "... and that each should work to transfer the same homilies into rustic Romance or German language [rusticam romanam linguam aut thiotiscam], so that all can more easily understand what is said"
"One effect of Alcuin's reforms must ultimately have been to impress on everyone that Latin, as written and spoken, was actually now a foreign language, not just the written, quasi-eternal form of Romance speech.
Ostler also observes that later, in Dante's time (1303), it had been quite forgotten that the Romance languages even derived from Latin (or indeed anything else); Dante was controversial for suggesting otherwise (p. 176).