The tendons that control movement of your fingers normally slide smoothly through the sheath that covers them, lubricated by synovial fluid which is a slippery substance produced by the tendon sheath lining (synovium).
Trigger finger normally occurs as the space for the tendon to glide gets smaller. This can be due to the pulley (like a belt loop holding the tendon to the bone) getting narrowed or due to the lining of the sheath becoming thickened. This produces a lump on the tendon which catches and gives the characteristic catch or triggering. Do note that it is possible to have a trigger finger without it catching. In this instance you would find pain and nodularity.
Trigger finger is more common in older people and occurs more frequently in women than in men.
Conditions like diabetes, rheumatoid arthritis and gout can increase the risk of trigger finger due to their tendency to cause thickening of the lining. If you have surgery for carpal tunnel syndrome you are at greater risk of trigger finger in the first six months after the operation.