p. 71 "He took a step, able at last to hit someone, stuck his fist into Piggy's stomach."
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Roger and Maurice are the ones that attack Jack and Piggy because Jack wants Piggy's glasses.
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.
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.
Piggy and Jack are complete opposites so they were bound to but heads. In the beginning Jack and Piggy didn't have a problem with each other but as the story progressed the more Jack hated Piggy. It never really stated why they don't get along but I think that it's because Jack has a different personality. Jack is power thrived while Piggy is more intellectual. Hope that helped!