Originally Posted by cds43016

Just from what I read, somehow in the process a primer is pinched or crushed in the press. It could be from trying to insert a primer in a case that already has one, trying to put large primer in a small primer pocket (you have to love the guy that decided to put small primer pockets in 45 ACP), priming a case that has a primer crimp that wasn't removed, trying to load a sideways primer or primer that just gets somewhere where it shouldn't. The list can go on. I guess whether it goes off depends on what's happening and the force applied. It also may depend on the primer brand being used. Some are more sensitive than others. There is also some luck (or bad luck) involved. The key is how the press reacts and how the force is contained.