A lot of things are "possible".
Working with facts and not possible/guesses is how you make improvements or reduce risks.
Every cartridge will have internal pressure rise when there is bullet setback and reduced internal capacity. That is not exclusive to .40 S&W or any other cartridge.
Going back to square one, loading wise, as mentioned earlier in the thread and inspecting your reloading procedure and equipment is generally a good starting point. Making sure the rounds are being produced with clean equipment is part of that procedure. Inspecting the firearm and brass may tell you other things.
The fact is that unless a person has the actual firearm as well as the brass at hand, trying to determine a cause as to why what happened, happened, it is pointless, and people are just making unsubstantiated guesses. Usually those are based off of "I read this".
I have read a fair number of articles by guys in magazines/online articles and I will say that a substantial amount of them are either completely making crap up, or they are cut and pasting their "expertise" from somewhere else and a good bit of it is simply wrong. That information then gets repeated on online forums. "Joe gunwriter says that .44 Magnums are not lethal past 75 yards, and you can't hit anything with a handgun at 100 yards".
Agreed 100% - one point about setback is, it should be fairly consistent in a given batch of loads. If the OP still has some of the loads in question, they can be checked for setback, just cycling through the action without firing.
