I seem to be greatly in the minority here (a familiar position!) but IMO if someone sets off the powderkeg without warning it is - or should be - that act of setting it off that starts proceedings. This means that either the setter-off should act first if most or all others are surprised*, or should get a whacking big bonus on initiative if they are not.
I fully understand the logic of this, and if I'm imagining a scene in a book or a movie then I completely agree with you.
However, in talking about a game I care for the playability of the game as much (or more so) than the realism. And, as I and others have explained, if you start allowing narration to supersede rules, there's no clear boundary of where it ends.
And, in terms of resolving the disconnect between the rules and the narration, one simply has to allow that sometimes things unfold in unpredictable ways.
Player: "I pull the trigger on my crossbow."
DM: "Ok, everybody roll initiative."
Player: "Wait! All I did was move my finger a centimeter! I should get that attack for free."
DM: "Well, that's what you
wanted to do. Let's see what actually unfolds..."
(everybody rolls, player with crossbow rolls poorly)
DM: "Ok, what just happened?"
Player: "Arrrggghh! My crossbow jammed!" Or, "In my excitement, I forgot to put a bolt in place."*
*Alternately: "My suit didn't come back from the cleaners! A friend came in from out of town! My car broke down on the way to church! IT WASN'T MY FAULT!"