A lot of clients I work with in our Boulder Physical Therapy practice report wrist pain in their handstands. While there can be multiple contributors to wrist pain, including a loss of upper body mobility or strength, one of the most common issues I see is lack of overhead shoulder mobility. When the arm is unable to get into an overhead position it results in compensation with extra motion and load at the wrist:
PICTURE1
Closed shoulder position with increased wrist extension
PICTURE2
Open shoulder position with regular wrist extension
Here is my favorite drill to work on overhead mobility for the handstand: