Causes
There is a continuing debate as to the reasons for the vowel shift among experts in linguistics and cultural history. Some theories give the cause as the mass migration to southeast England after the Black Death, where differences in accents led to some groups modifying their speech to allow for a standard pronunciation of vowel sounds. The different dialects and the rise of a standardised middle class in London led to changes in pronunciation, which continued to spread out from the city.
Another theory is that there was a sudden social mobility after the Black Death, with people from lower levels in society moving to higher levels (the pandemic also having hit the aristocracy). Another explanation highlights the language of the ruling class: the medieval aristocracy had spoken French, but by the early 15th century they were using English. This may have caused a change to the "prestige accent" of English, either by making pronunciation more French in style or by changing it in some other way, perhaps by hypercorrection to something thought to be "more English" (England being at war with France for much of this period). That rationale is disputed because there is just as much evidence of the hypercorrection to be "more English", as there is for it to be "more French" (French still being the slightly favored language of the upper class). Another influence may have been the great political and social upheavals of the 15th century, which were largely contemporaneous with the vowel shift.
Read more about this topic: Great Vowel Shift