That is some weird coincidence or you psychically fired the gun.
Also if you "had this gun built some 15+ years ago, Douglas barrel, Shilen trigger, McMillian stock" then your statement as to "I have done no trigger adjustments or anything to the gun" is way off. Your post doesn't exactly make sense.
My guess, if it happened as explained, is you have 15+ years of grit and oil in that Shilen trigger and the sear /weight set too low. Good luck.
