What Causes Hyper Reflexes In The Knees?
If only knees not due to disc disease.
Detailed Answer:
I read your question carefully and I understand your concern.
There are some variations between individuals in their reflex activity, it is possible to be born that way. That is why we do not make diagnoses based on isolated findings but considering the whole.
Hyperactive reflexes in general are caused by central nervous system (brain and spinal cord) lesions like multiple sclerosis, tumors, stroke etc as well as from metabolic issues such as high thyroid function or electrolyte abnormalities. However the later generally increase all reflexes, while brain and spinal cord lesions alter reflexes in areas corresponding to the damage.
In your case increased reflexes only in the knees doesn't correspond to brain or spinal cord lesions, brain lesions cause symptoms on half of the body, while spinal cord lesions cause symptoms below the lesion so wouldn't affect only the knee but at least the ankle jerk reflex as well. In addition there are usually other signs such as weakness, altered sensation, bladder control issues, balance issues etc. That is why it is unlikely that your hyperactive reflexes represent any brain or spinal cord lesion.
As for the cervical disc degeneration that would cause increased reflexes only if there are disc bulges or bony spurs compressing the spinal cord, the degeneration itself doesn't do that. But even if there was spinal cord compression the reflexes would be increased below the compression, hence in all four limbs, not only in the knees.
I hope to have been understandable, I remain at your disposal for other question.