I took the shotgun to two gunsmiths today. Both had the same opinion. (The first sent me to the second to confirm.)

Slam fire.

The shotgun was set up to fire 3½ shells from the factory. My wife was shooting 2 3/4 light loads. The main spring was at a VERY heavy setting, and the firing pin spring is very light. When the bolt slammed shut, the firing pin still had inertia that its light spring could not resist and it kept travelling forward, detonating the primer.

Fortunately, the solution is easy. As the Spartan 453 has an adjustable gas system, I simply decreased the tension on the spring.