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!
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After the signal fire has gone out, as a result of Jack 'needing' the boys who were on fire duty to join his pig hunt there was a confrontation on the mountain top. Ralph told the boys that they had missed the chance of being rescued by a passing ship because the fire was out. Jack managed to accept criticism from Ralph but when Piggy also spoke against him Jack had an easy target on whom to vent his frustration and anger. So he punched Piggy in the stomach, which made the fat boy sit down with a bump. Then Jack slapped Piggy's head, which send Piggy's glasses flying and resulted in one of the lenses breaking.
At the beginning, Piggy told Jack that he should not have let the fire out, since it was his responsibility. Some of the hunters agreed with him, but this caused Jack to become malevolent. He hit Piggy in the stomach, making Piggy sit down with a grunt. Then Jack tells Piggy that Piggy woulda done the same. Then Jack takes it too far, and slaps Piggy's head, causing his glasses to fly off. One side of the glasses broke, and piggy yells at Jack saying " i got to have my specs. Now i only got one eye. Just you wait...." Piggy repeats this 3 times, when they are separated between a great rock. They continued to scramble around the rock until Ralph breaks in.
Piggy is a fountain of ideas and logical reason but realises he completely lacks the influence to get the group behind him. Jack has that influence, and thus has a growing tribe of savages. Piggy thus fears Jack on two levels: 1) because he represents a power that will bring Piggy's remembrance of civilization and order down; 2) he and his followers present a physical threat to the weak and outnumbered Piggy - another way the book shows that ideas, in times of anarchy, don't matter as much as who has more strength.
Jack refuses to give any meat to Piggy to reassert his own power, after being told off by Ralph.
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.
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.