I have had a very similar experience loading 40 S&W brass that was "past its prime". In the first place 40 S&W brass is thin-walled out of the factory. I was shooting USPSA Limited class and loading the ammo with 220 gr lead truncated cone bullets. Powder charge was a very moderate amount of Clays. I had two events of the case head completely separated from the brass in the chamber during a stage(this really ruins my score since the gun is out of service). I was able to remove the brass at a safe table and rejoin my squad(a bit apprehensively). This happened to me once more before I discontinued using that load in matches. There was no question of double charge and I learned that others had similar problems.

Old brass and Clays OR Titegroup is trouble looking to happen without a double-charge. I now load 200gr bullets with a bit of N320 in once-fired police brass and leave it on the ground. Starline is the only 40 brass built with stronger side walls.

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Bob.
SWCA 1821