The Great Vowel Shift: modern English

There came a period from about 1500 to 1800 when the English language changed quite dramatically in quite a short space of time. Vowels became shorter creating a distinct change in the pronunciation of many words. Because from the end of the 16th century, the British had contact with many people throughout the world, meant that new pronunciations and phrases began to be used. The invention of printing meant that spelling and grammar became more fixed as manuscripts began to be made available to the masses. In 1604, the first English dictionary was published. It was compiled by Dr. Samuel Johnston. All the printing houses were in London, meaning that this became the standard dialect of the masses.
Sources: http://www.englishclub.com/english-language-history.htm