What are the causes of hypertrophic changes in facet joints of the cervical spine?

Inflamatory arthriti: Rheumatoid arthritis, trauma, are a few causes among others, osteoarthritis advanced age.

Your age, smoking: Aging, smoking, prior neck injury all can be a factor of overgrowth of the facet joints which is a sign of degeneration. Whiplash injuries in over 50% of cases involve some facet joint component that can accelerate degenerative changes that occur normally with age. Smoking accelerates this degeneration by affecting the rate the discs degenerate causing narrowing & secondarily affecting joints.