Because he's a troll
Jack screams this because he feels threatened by Ralph questioning his ability to hunt and provide for the group. Jack's identity and power are tied to his role as the leader of the hunters, so he reacts defensively when Ralph challenges him in front of the other boys.
That Ralph is still chief.
Ralph
Ralph escapes the hunters while stabbing two boys in Chapter 11 of the book "Lord of the Flies" by William Golding.
When the boys elect Ralph as leader, in favour of Jack, Ralph puts Jack in charge of the choir and asks him what he would like them to be. Jack replies, "Hunters."
Ralph is elected as the chief but Jack assumes the role of chief of the hunters.
The main characters of Lord of the Flies are Ralph, Simon, Jack, The Hunters, Piggy, and Samneric. The other characters are The littleluns, Maurice, Robert, Percival Wemys Madison, Johnny, and Henry.
Ralph tried to decide which option was best, either climb a tree and hope not to be spotted, burst through the line of hunters or hide and hope that they would pass without finding him.
In chapter 8 of "Lord of the Flies," Ralph characterizes Jack's hunters as primitive and barbaric, with an emphasis on their violent tendencies and detachment from civilization. He sees them as being consumed by their bloodlust and becoming more savage as they embrace their roles as hunters. Ralph is disturbed by their descent into savagery and the way they prioritize hunting over maintaining the signal fire and runway.
He thinks about washing his dirty grey shirt.
In Chapter 4 of "Lord of the Flies" by William Golding, Ralph is mad at Jack because the hunters were supposed to maintain the signal fire. The hunters barely hear or react to Ralph's tongue lashing, more distracted by the joy that they actually caught a pig.
Ralph decided that he could either hide and hope that the hunters went past his hiding place without spotting him or he could break through the advancing line of hunters, hopefully without being spotted.
In William Golding's novel "Lord of the Flies," it is Jack's group of savages who accompany him to steal fire from Ralph's camp. They violently attack Ralph and the other boys to steal the fire and assert their control over the island.