When looking at examples listed in OED it is very noticeable that English differs greatly before 1700s and after (roughly) and it becomes recognizable and very similar to modern starting roughly from 1700s. What is behind this transformation? What happened in society which transformed language?
I would venture it probably has a fair amount to do with Samuel Johnson's Dictionary of the English Language, published in 1755.
Basically, the long vowels shifted upwards; that is, a vowel that used to be pronounced in one place in the mouth would be pronounced in a different place, higher up in the mouth.