During/after physical therapy for my wrist my pinky/ring finger started shaking. what could be the reason?

Overuse tremor: Likely that the demanding exertion in your therapy sessions, particularly when exercising the muscles in your hand, result in residual spasm - shaking in the finger. Benign & not a health risk. I'm assuming though, that your wrist problem has no related nerve injury, since you didn't mention that. Hope that helps.

The ulnar nerve: passes through a tight canal at the wrist. Movement of the wrist, particularly an injured wrist can result in inflammation around the ulnar nerve causing some compression, or the nerve itself can become irritated or inflamed. Irritation or inflammation of the ulnar nerve or surrounding tissues at the wrist or elbow can result in numbness, tingly, and/or trembling of the pinky finger.