What causes carpal bossing?

Inflammation: Inflammation at insertion of extensor carpi radials brevis an longs at base of second and third metacarpals results in periosteal inflammation and heterotopic bone formation causing the boss to form.

No one knows: Carpal bossing is probably a variation of degenerative joint disease affecting the second and third carpal-metacarpal joints resulting in small osteophytes or extra bone arising from the dorsal aspect, resulting in a focal bump which can be felt on examination. They are usually not clinically important, but may cause pain if the back of the hand strikes another object at that site.