Why did Jack attack Piggy in the book Lord of the Flies?

After the fire was allowed to go out and the boys missed an opportunity for being rescued Jack managed to accept criticism from Ralph. However when Piggy also spoke against him Jack had an easy target on whom to vent his anger and frustration. Jack 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.