The vowel sounds shifted upwards in the mouth.
The greatest period of change was from the 1400 to 1600's. Had the printing press come around 100 years later, English spelling would be much more logical.
No one knows why -- or exactly *how* --the Great Vowel Shift happened. It didn't happen in any other language, although a consonant shift occurred in German in the 600's.
Vowels are still shifting today, as evidenced by words like "route."