The gun was in the holster, and the entire holster was removed from the belt and placed on a desk. There's nothing in that course of action that would allow a finger or anything to get into the trigger well. The gun should absolutely not have gone off.
A gun that can go off on its own even when handled properly is the opposite of idiot-proof.
The Air Force is not. (The average Air Force servicemember is older, better educated, better paid and subject to more demands than the average servicemember in the other branches.)
Given a large enough group of people you will have examples of just about anything (including idiots)
Another example, it's estimated that 1-2% of all military members are part of or connected to an illegal gang organization.
Every population has some idiots in it. My point is the Air Force is difficult to get into, has a high fraction of educated servicemembers [1], has lower turnover and higher pay than the other branches. (Back when we measured IQ, the Air Force had a higher average IQ than the other branches and population.)
I know idiots who work at Google. That doesn't it's safe to assume the average person at Google is an idiot.
[1] https://www.collegevine.com/faq/90807/most-educated-branch-o...
Historically the word "idiot" was used particularly with those with very low IQ scores but now is a pejorative term.
(Is easier to sit in.)
Obviously that's not always practical, but if you are placing a chambered firearm down on a desk, you might as well try to point it in the direction of least harm, it basically costs zero to do.
If they can prove this in the investigation, this completely sinks Sig's defense that this can't happen with the upgraded FCU that they released to supposedly fix this issue since it's in all military variants.