Well, we can "debate" this situation endlessly, and still won't know exactly what happened until ... IF ... the investigating agency, or the involved detective's agency, decides to clarify what happened in a statement.
I'd "suspect" ... and it's only that, mind you ... that the ND involved either a holster condition issue ("... and adjusted the gun as he sat down."), or else the detective's actions unintentionally caused the trigger to be depressed sufficiently for the weapon to fire, such as "something" entering the trigger guard far enough and depressing the trigger while he was adjusting the weapon ... and it might've even been his finger.
I've watched folks on the range unintentionally, and apparently completely unaware, position their index fingers partially inside their trigger guards while reholstering.
We might never know ... but it's certainly a potentially grim reminder of how we must always be aware of what's happening with our weapons, and make sure our preferred methods of carry don't present potential problems, either by design or by improper user actions.
This could "happen" to any of us, under the wrong conditions ...
Use caution ... always.