Jack punched Piggy in the stomach, which caused him to sit down with agrunt. Then Jack stood over him and smacked Piggy's head, this send his glasses flying and one lens broke, as they landed on the rocks.
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Roger and Maurice are the ones that attack Jack and Piggy because Jack wants Piggy's glasses.
For Piggy's glasses, which means the infinite power of fire.
Simon retrieved Piggy's glasses for him, after they had fallen onto the rocks as a result of Jack punching Piggy.
Jack attacked Piggy rather than Ralph because he knew Piggy wouldn't fight back. If he had attacked Ralph there was a good chance that Ralph would have beaten him and Jack couldn't take the risk of losing face in front of his own hunters.
After Jack leaves to go to Castle Rock, Piggy becomes more confident with himself, and starts to stand up for himself. This happens because Jack stole Piggy's glasses, and now he can no longer hide behind his disability of not being able to see.