The scar was caused by the plane crashing.
The boys are on the island because their plane crashed. They were being evacuated from the war, and their plane crash-landed on the island. All adults were killed. There are several descriptions of the crash site, and it is called the “scar” because it must have scraped when it crashed.
Beyond falls and cliffs there was a gash visible in the trees; there were the splintered trunks and then the drag, leaving only a fringe of palm between the scar and the sea. There, too, jutting into the lagoon, was the platform, with insect-like figures moving near it. (Ch. 1)
Calling the crash the “scar” is just another way of ignoring reality. The boys never talk about the crash, or the world they left behind. They occasionally talk about rescue as an abstract concept, but they never talk about the plane itself after the initial crash.
The only time the plane crash or the cause of the scar is mentioned is in the very beginning of the book.
“When we was coming down I looked through one of them windows. I saw the other part of the plane. There were flames coming out of it.”
He looked up and down the scar.
“And this is what the cabin done.” (Ch. 1)
After this, it is as if reality takes a backseat to the reality that the boys build for themselves on the island. They develop their own civilization, apart from the one of the adult world, and it slowly breaks down into a savage mess until two boys are killed.
When the boys are finally rescued, all Ralph can do is cry when he finally sees an adult. There is no way for him to explain yet what happened to those boys on that island. Order descended into chaos so quickly.