“Wait a minute though! Where does the pig-run go to?”
“The mountain,” said Jack, “I told you.” He sneered. “Don’t you want to go to the mountain?”
Ralph sighed, sensing the rising antagonism, understanding that this was how Jack felt as soon as he ceased to lead.
There's the bit of the novel you need. Ralph is sensing the "rising antagonism" - that is, Jack's growing unwillingness for Ralph to be chief, and Jack's growing desire to lead himself. "... this was how Jack felt as soon as he ceased to lead". It's an early indicator from Golding that Jack not only resents Ralph, but already has thoughts of being in charge himself. And though Ralph recognises it, he doesn't act decisively enough to prevent the inevitable: Jack challenging his authority on the island.
In chapter 7, the biguns go looking for the beast that Samneric saw on the mountain. Ralph leads the way, but as daylight fades, he realizes that the hunt will have to be postponed. In the dark they won't be able to see the beast, and he doesn't want to leave Piggy alone all night with the littluns. So he suggests going back to the shelters and continuing the hunt the next day. He is trying to be logical and use reason as his primary leadership method.
Jack takes the opportunity to mock Ralph. "We mustn't let anything happen to Piggy, must we?" he sneers. Ralph ignores the taunt, still trying to problem solve. Simon volunteers to go back, and Ralph sees Jack "infuriatingly, for the first time." Whether it is Simon's willingness to provide meaningful assistance, or Jack's sneering about Piggy, or both--something gives Ralph insight about Jack that he hasn't had before. He then grills Jack for more information about the pig run. If Jack had been using his head and if he had been willing to help Ralph rather than trying to stymie him, Jack would have realized earlier that they could have been following the pig run and would have made better time on their quest rather than struggling through impassable parts of the forest. Ralph tries to get more information from Jack about where the pig-run leads. Again, he is merely trying to stay on task. But Jack sneers again, insinuating that Ralph is scared to go to the mountain.
Ralph senses that Jack is angry at him. He realizes that whenever Jack isn't top dog, he is snarly and unpleasant. Jack snaps at Ralph again, taunting him about Piggy. Ralph recalls that Piggy had told him that Jack hates Ralph, and in that moment he realizes the truth of Piggy's opinion. He asks Jack directly, "Why do you hate me?"
The conversation about the pig-run reveals to Ralph that Jack isn't thinking about the mission or the good of the group; he is angry about not being the leader of the boys, and he hates Ralph. Ralph is "hot and hurt," responding to Jack with anger but feeling the loss of what he had believed was friendship between the two of them. He begins to realize here that Jack is as dangerous as Piggy predicted.