Lifting babies and young children puts you at risk for this problem. You are grasping, holding, and putting pressure on the wrist while it's in a downward angle.
These hand and wrist positions and motions cause irritation of the tendons at the base of the thumb. Since you perform these movements many times each day, they become repetitive. The constant tension on the tendons causes inflammation, thickening of the tendons, and swelling around them.
New mothers are especially at risk because it's a new movement and it's often done while the child is moving. Hormone fluctuations (up and down) linked with pregnancy and nursing may also add to the risk of developing this problem.