In adults, cervical instability most commonly results from
Cervical instability, defined as AP subluxation of more than 3.5 mm or sagittal angulation greater than 20deg on flexion-extension radiographs, most commonly results from disc degeneration, with its attendant changes in the facet joints and loss of ligamentous suppo. Ref: internet sources
A rotational injury
A flexion extension injury
Degenerative disc disease
Increased ligamentous laxity
Spinal injuries