The Great Vowel Shift has been attributed to many factors in a never-ending debate of linguistics over the underlying cause of the GVS. The theories, being arguments pushed off previous theories can be grouped to some extent, arguing for, or against other past theories. These groups of theories argue occurrence through regional issues, mass adoptions of foreign lexicons, chain shifting, and to suit the ever-changing English language, and society. Of the previously listed groups of theories, the most plausible in my mind would be in concerns with the massive influx of loan words.
One such of these theories states that the Great Vowel Shift occurred “due to an influx of Old French loan words coming into adoption into Middle English.” This theory has proof lying in such problems as the failed union of /e:/ with /:/. “The diphthongization of Middle English /i:/, the alternation between /o:/ and /e:/, as well as the loss of inflectional morphology. The loss of unstressed final syllables may be connected to the changes in long stressed vowels.” (Bertacca, Antonio. The Great Vowel Shift and Anglo-French Loanwords: A Rejoinder to Diensberg 1998. (2000))
Another similar theory is that the loan words from Romance languages during the Middle English and Early Modern English helped to shape the English vowels during this time. Evidence is given through variation in vowels of Present Day English created by the Middle English period’s stressed vowels, “particularly the long high mid & low mid vowels & diphthongs ending in –i”. Such changes were also shown in Gallo-Romance loans, “many of which are conditioned by tonic vs pretonic position in the source dialects”. Dialects in Old French are shown with the creation of five “competing English phonological systems in the late medieval and modern periods.” The raised e in Middle English, which spread e to native lexicon, is noted as characteristic of French loan words thought to be similar; and similarly the movement of /e:/ to /i:/ (Interestingly enough predating the Great Vowel Shift) “ spread from a corresponding alternation before s, v, & r in French loanwords.” (Linguistic Change in English: The Case of the Great Vowel Shift from the Perspective of Phonological Alternations Due to the Wholesale Borrowing of Anglo-French Loanwords. Diensberg, Bernhard, Folia Linguistica Historica, (1998))
These are the theories I agree mostly upon due to changes of English language of that time influenced greatly from such languages that were thought to be of more prestige. Eliminating final vowel, inflections, and many other influences have shaped English from these languages, which is why I believe this is the most notable of the theories of the Great Vowel Shift.
Other such theories involve regional issues; these are evidently real through the irregularly changing dialects of Middle English, and Early Modern English. One issue is that of the Black Death occurring in England at the times of the Great Vowel Shift. This epidemic was cause for many Englanders to move South East in search of safe haven from the plague. Hereby, differences in accents led certain groups to modify their speech patterns to allow for standardization of vowel sounds. The different dialects and the rise of a standardized middle class in London led to changes in pronunciation, which continued to spread outward from London.( www.answers.com/topic/great-vowel-shift)
Another regional theory argues that “shifts in northern, midland, and southern regions must be considered separately and that the GVS in the south is not a unitary phenomenon.” The movement of mid vowels is thought to be caused by “crowded front zone” with the north, and the “sparsely populated back zone” has little reason to form shift. The south forms two systems from the release of the ending –e. “System 1, with three long mid vowels, was used by high social circles represented earlier by Geoffrey Chaucer; system 2, in which lengthened e merged with /:/, was less prestigious. System 2 speakers, who perceived the high mid vowels of system 1 as high vowels, tended to raise the low mid vowels & favor diphthongization of high vowels (originally a relaxed-speech variation). Phenomena of social distancing, reactive hypercorrection, & East Anglian immigrant speech are additional factors; affected speech of seventeenth-century descendants of system 2 speakers completed the GVS with the raising of System 1 /ae:/ to /e:/.” (Dialectal Variation in Middle English and the Actuation of the Great Vowel Shift. Smith, Jeremy J)
Another theory notes that when breaking the region in terms of when the Great Vowel Shift occurs, it is possible to break down into “Northern varieties & for Southern varieties” as well as “Standard as well as most Southern & Midland varieties” as the elapsed time between the change varies. Evidence is supported through text from the medieval period “and by Yorkshire dialect evidence”. (English Vowel Shifting: One Great Vowel Shift or Two Small Vowel Shifts? Johnston, Paul A , Jr.)
The chain rule (which seems to be the most commonly thought of and used) I find has a flaw. It lacks the account of dialect variation in terms of the Great Vowel Shift. This is the means of argument from more recent theories, challenging the chain shift claiming “evidence” supporting over chain shift. Arguing that the GVS occurrence is through the initial “raising” of a vowel in the pull theory, stating that the /i:/ and /u:/ shifting, giving way for lower vowels to move up in articulation. (A Biography of the English Language. C. Millward). Also argued is the push theory, similar to the pull theory only the initial movement comes from a lower vowel “pushing” rather than “pulling” the following vowels.
While some argue for physical reasons, causing the Great Vowel Shift, others argue in terms of social reasons that the GVS occurred during the rise of the English language, enhancing it.
Giancarlo, argues that the social era effects the formation of the Great Vowel Shift, with its representation of gradual change of “phonetics reinforcing belief in cultural progress, the presumed inferiority of nonstandard English dialects, and the problematic identification of Shakespeare as a modern writer.” Also claiming the GVS a representation of a “boundary between demands of scientific visualization & basic assumptions of cultural representation.” (The Rise and Fall of the Great Vowel Shift? The Changing Ideological Intersections of Philology, Historical Linguistics, and Literary History. Giancarlo, Matthew).