When told I have trigger finger, I was at a loss as to how it could have started. What usually causes a problem like this?

Triggering is usually the result of a thickening in the tendon that forms a nodule, or knob. The pulley ligaments that forms an arch over the tendon may thicken as well. The constant irritation from the tendon repeatedly sliding through the pulley causes the tendon to swell in this area and create the nodule. Rheumatoid arthritis, partial tendon lacerations, repeated trauma from pistol-gripped power tools, or long hours grasping a steering wheel can cause triggering. Infection or damage to the synovium causes a rounded swelling (the nodule) to form in the tendon.