How would you rate your backswing tempo to the top of the swing? If it is rather fast and you must come to an abrupt stop at the top before you reverse into the downswing, you could be creating a lot of pressure on your thumbs as they form a fulcrum under the club (Form I Lever). Alternatively if you are a hitter with a shortened backswing, then you may well be aggressively reversing the club, resulting in extra thumb fulcrum pressures.
I just can't see how your thumbs can get involve with the club in the downswing and through Impact. Perhaps brainman could help me on this one.
Btw, when brainman asked you how long was your thumb, my first thought was that he was inquiring if you held the club with a long or short left thumb position on the handle at Address. Do you?