I believe brass quality is becoming an issue.
I've had this issue with some very good Swedish and Finnish brass and ammo from Europe in three rifles from Europe and one a custom....some of the brass was new, some from the 1970's.
It can happen at any time.
Should note I had a headpsace gauge; 99.9% were fine, but since I had about 2200 cases, I did find the offenders after loading all up and getting occasional clicks. It wasn't any of the rifles.
