In my experience, the main factor is not thinking, training or circumstances, the main factor is willingness– specifically, willingness to believe.
The next factor, as much as everyone hates to admit it, is luck.
There is a moment, when something happens, where one "can't believe this is happening". Mental inertia. Partly because one is unprepared to believe it's happening, partly because one doesn't want to believe it's happening. The hesitation between the stimuli and the reaction is personal, and I would never question another person's reaction to combat.
When one overcomes that, the next factor is luck. Millions of people have ducked left, early and well, straight into a mortar round. A bad guy spraying bullets down a hallway is equally unpredictable. If it were possible to see and avoid bullets, that's what we'd teach.
Mental preparation helps overcome inertia. Training helps with the mechanic skill. Sometimes the combination wins fights. And sometimes the highest skilled, hair triggered rifleman dies.
It's much easier to say what won't happen.
If you don't drill, you won't win because you won't hit the target.
If you don't think about it, you won't win because you will hesitate.
"Huh?" --Jammer Six, 1998