Unilateral hoffman's sign and on the same side have non sustained ankle clonus. what's the cause of this?

Probably nothing: Unsustained ankle clonus and one-sided Hoffman's side may be normal for you. Especially if the rest of your neuro exam is normal. Many healthy people have several beats of clonus in one or both ankles. Concerning would be jumpy reflexes in arms and legs on the side with the ankle clonus. If that were the case, then all these together would be very concerning. Best to see a doctor for exam.

Poss c-spine prob: Hoffman's sign is used by clinicians when there is a suspicion of cervical spine pathology possibly pressing on the spinal cord. The sign is more accurate when present bilaterally but can be seen when unilateral. It has also been reported in hyperthyroidism and anxiety states. I hope this helps. Good luck and well wishes.