A.C. Baugh (The History of the English Language [1993] 20) explains the reason for the Great Consonant Shift this way:

It is often assumed that the change was due to contact with a non-German population. The contact could have resulted from the migration of the Germanic tribes or from the penetration of a foreign population into Germanic territory.

Or it could simply be because "languages drift," an enduring but inexplicable fact of linguistic history. In some ways, linguistics is no more of a science than the study of changes in clothing styles or interior design, where cause and effect can sometimes be extrapolated from the evidence but all too often changes are, in fact, the product of whimsy and fashion.

The vowels in Common Germanic probably also shifted at this time, but vowel change is much harder to track than consonant shifts—vowels are naturally more fluid than consonants—and so it's not possible to speak about the vowels specifically.

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