If you're speaking about the meeting in chapter 8, it's actually Jack who calls the meeting. At least, he's the one who blows the conch and hijacks Ralph's authority in leading the assembly. It results from an argument between the two boys, culminating in Ralph insulting the hunters. This, for Jack, is the last straw.
The sound of the inexpertly blown conch interrupted them. As though he were serenading the rising sun, Jack went on blowing till the shelters were astir and the hunters crept to the platform and the littluns whimpered as now they so frequently did. Ralph rose obediently, and Piggy, and they went to the platform.
“Talk,” said Ralph bitterly, “talk, talk, talk.”
He took the conch from Jack. “This meeting—”
Jack interrupted him. “I called it.”
“If you hadn’t called it I should have. You just blew the conch.”
“Well, isn’t that calling it?”
So Jack has taken the first step in undermining Ralph's authority: he's taken control of the conch. During the meeting, he gives several reasons for why he believes Ralph isn't a proper chief. The first is that Ralph is too much like Piggy. Jack doesn't respond to logic and rational thinking, & building shelters and keeping the fie going are too much for him. The second reason is that Ralph is a coward in Jack's eyes. He says that Ralph didn't go up the mountain with he & Roger, even though Ralph did. When Ralph argues this point, Jack lets loose a tirade.
“He’s not a hunter. He’d never have got us meat. He isn’t a prefect and we don’t know anything about him. He just gives orders and expects people to obey for nothing. All this talk—”
“All this talk!” shouted Ralph. “Talk, talk! Who wanted it? Who called the meeting?”
It's obvious that while Ralph is thinking clearly about Jack's reasoning, Jack is not. He's upset, he wants power, & he's determined to wrench it from Ralph.