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.
It shows that Jack rules by fear and violence.
Jack initially dismisses Piggy and the influence he has on the group. He belittles Piggy's intelligence and continually undermines his authority in order to maintain control over the other boys on the island. Eventually, Jack's treatment of Piggy escalates into physical violence, culminating in Piggy's tragic death.
Jack is feeling guilty just as Ralph and Piggy are.
After Ralph slashes Piggy with water, Piggy yells at him for being unkind and immature. Piggy also criticizes Jack for becoming ruthless and embracing violence as a leader.
Jack took Piggy's glasses.
Jack and his followers steal piggy's glasses.
Ralph and Piggy confront Jack to try and reason with him and bring the group back together. Their plan fails because Jack is consumed by power and control, and he refuses to listen to reason or cooperate with them. Jack's actions ultimately lead to chaos and violence within the group.
If you are asking "Why did Jack hit Piggy?" The answer is that Jack was elated from having just successfully hunted a pig for the first time. He felt on top of the world, he felt that he had proved his skill and ability before his hunters. Then his self congratulations had been brought tumbling about his ears when Ralph told all the boys that they missed the opportunity of being rescued because the fire had been allowed to go out. This was a direct result of Jack 'needing' the two boys who should have been on fire duty to join his hunt. While Jack could grudgingly accept criticism from Ralph, who was bigger than himself and also the elected leader, when Piggy also spoke out against him it was a step to much. Jack could not accept losing face before his hunters by allowing a fat and unpopular boy to verbally chastise him without response. So, Jack punched him. Piggy was an easy target who was unlikely to fight back and who was unlikely to attract too much support. Jack was therefore able to unleash his anger and also the frustration he felt towards Ralph but which he was unable to express in that direction.
Roger and Maurice are the ones that attack Jack and Piggy because Jack wants Piggy's glasses.
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