I am NOT a doctor.....but IF I was, I would not rule BCG treatments out as a cause for trigger fingers. As a less common side effect, Medac, a supplier of BCG for intravesical treatment of bladder cancer in Europe, is listing joint inflammation (arthritis) and joint pain (arthralgia) as less common possible side effects. Trigger fingers could be caused by rheumatoid arthritis. Rheumatoid arthritis is an autoimmune disease, so Medacs statement make sense to me, as the BCG treatment is suppose to boost the immune system.
I have not got BCG myself, but what I have read, almost everyone get some side effects! But, for most people, it is worth it, as it is the best available treatment.
link to the Medac info (with google translate):
translate.google.se/translate?hl=sv&sl=sv&tl=en&u=http%3A%2F%2Fwww.fass.se%2FLIF%2Fproduct%3FuserType%3D2%26nplId%3D20020419000099