What is the trigger incident in the novel, Lord of the Flies?
I am sure it will depend on who is answering the question. In my opinion there are two places that you can really point to as trigger points.
One is when Simon rushes out of the woods after having his vision and passing out right into the midst of the killing dance of Jack's tribe. The boys fear he is the beast and in the incredibly terrifying atmosphere of the raging storm in the dark, they stab and kill Simon, the first time they have killed someone, though one of the little ones may have died in a fire at the beginning.
But another place some people could point to is when Roger tips the rock over to kill Piggy because this is murder without any kind of confusion, he decides to push the rock over and kill him indicating that there is no longer any barrier to murder among the boys.
When you ask about the "trigger incident" I believe you are asking about the inciting incident.
This refers to the act or event that puts a story in motion. In Lord of the Flies, the action of the story would not have happened if the plane was not shot down and the boys didn't end up on a deserted island. These events trigger, or set this story in motion. Had their plane ended up at their chosen destination it would have meant the story could have taken an entirely different route.
The inciting incident is important because it helps determine the resolution and even the climax of the story. The inciting incident will always introduce a problem that later, by the climax, will appear resolvable or not. When it is or isn't, we have reached the resolution.