When I see a triggering finger or thumb with a normal looking A1 pulley it is usually due to a degree of tenosynovitis. When this occurs the tendon(s) becomes larger and then friction occurs and the friction exacerbates it making the tendon swell more etc etc and bingo you have a trigger.
Anything that leads to tenosynovitis will be the culprit. May be overuse, may be underlying rheumatoid. Diabetes is also a risk factor and no doubt sometimes it is just bad luck.
We generally treat these patients with a catabolic injection into the flexor sheath as a first up intervention.
Steve