With absolutely no data other than what you guys have already read..I have a bet regarding how this happened.

The H&K USP .40cal that pilot carried has a safety/decocker lever. It has a relatively "nubby" hammer. Manually pull the hammer back with a round chambered, and you're in a SA trigger mode, I think (at least in the variant I've handled). So the only safe way to decock is with the finger well away from the trigger and using the decock lever.

An old 1911 shooter, used to the old habit of riding the hammer down with the trigger depressed to "decock" the pistol is IMHO an AD waiting to happen with many modern pistols. On that H&K (and e.g. the SIG P6/225) I think you can very easily slip off of the hammer while pulling the trigger and trying to ease the hammer down. BANG. OOPS.

So..that's my bet about what happened.