From a description on the great vowel shift:

Some scholars believe the shift was influenced by the rapid migration of people from northern England to the southeast part of the country to escape the Black Death that killed over 25 million people across Europe. The migration resulted in the mixing of accents, warranting changes in the standard London dialect. Others argue the Great Vowel Shift occurred in response to an increase in the number of French loanwords used in the English language. Additionally, some claim the shift was caused by a hypercorrection in response to the increase in the number of French loanwords, which occurred either intentionally or unintentionally, to make English vowels sound less like French