We don't know. And it takes very little time for things to go completely pear shaped.
https://www.youtube.com/watch?v=W8oS1JuRxbQ
That took frickin' forever to go all the way south, in the terms of any sort of active encounter.
Dancing a jig was all the time in the world to draw and come on target.
Entering the truck was time to make sure the safety is off and get a finger on the trigger.
Weapon was visible for plenty of time to take the shot.
Essentially, this was the opposite extreme of the Levar Jones shooting. It seems police have great difficulty finding a happy medium.
Early on, I saw one article suggesting that the shooting was unintentional -- as in a nervous trigger press due to stress. That would indicate poor trigger finger discipline -- violation of Cooper's Rule Number 3 (the full version: "Keep your finger off the trigger until your sights are on the target and you're ready to fire").
I can't find that reference now -- I suspect the department has done their best to make it go away.
Because you can't justify an accident. Can't remember the name or place off the top of my head, but way back, there was a case where a guy was very justifiably aiming and ordering an attacker to stop while retreating. He would have been fully justified in shooting under the circumstances, but he claimed the gun went off when he stumbled. He ended up losing the case because he claimed it was an accident, not a completely justified use of deadly force in self defense.