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.
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Roger and Maurice are the ones that attack Jack and Piggy because Jack wants Piggy's glasses.
Piggy
Well in chapter 10 Jack and his tribe of savages invade Ralph's tribe (made of up Ralph, Sam and Eric, Piggy, and one/two littluns) in order to steal Piggy's glasses, which is overall the power to start a fire on the island. Jack might have mentioned this plan in chapter 9 so I'm hoping this is the right answer to your question.
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.
~SPOILER ALERT~ Jack doesn't do much to Piggy other than steal his glasses in order to make a flame. Early in the book, Jack hits Piggy and breaks one lens. Later on in the story, Roger, I believe his name is, kills Piggy by pushing a big rock off the cliff and it cracks Piggy's head and he drifts off to sea.