Cause of wrist subchondral cyst?

Cartilage: A subchondral cyst can be due to a congenital or acquired defect of the articular cartilage. These can be benign but are often associated with early degenerative changes. There is typically nothing you would do surgically to this.

Arthritis: Since you used the term subchondral cyst i assume that you have had an x-ray or mri. These can be caused by several things such as arthritis, old ligament tear, or possibly a ganglion that is inside the bone. Treatment depends on how bothersome it is, and ranges from simple things like a wrist splint, therapy, medication all the way up to major wrist surgery. Get it checked out by a hand surgeon.