It is hard for me to believe that an inefective seal between case neck and bullet, or around the primer, would draw enough moisture to cause that ? I will have to guess that after primeing, the case somehow got fouled with water, beer, pop,whiskey, solvent, or some other chemical found on loading benches ? If you used a powder measure with a drop tube, there can be another possibility. I once had a failure to fire in a 44 mag, loaded with 2400, which I had loaded the night before. While I was charging cases, my wife wife called me to supper. When I returned, I knew the measure was correct, so I just ressumed charging cases. After the miss fire, I returned home, pulled the bullet, and dumped the charge. There was still a wet clump of powder in the bottom of the case. I tapped the case mouth on the bench, until the clump fell out, and on the bottom was the remains of a rather large spider. Enough goop had sqeezed out of him to soak the primer, and about a fith of the charge of 2400. Since that time (about thirty years ago), if I get interupted while charging cases, I always dump the first charge back in the hopper, but have never had it happen again.
