First, we must understand the difference in leadership styles. Ralph became a leader out of circumstance. Ralph put the needs of the group above his own. Ralph admitted his own short-comings but committed to give the best of himself to the people he served. Ralph sought to introduce order by surrounding himself with an intellectual advisor in Piggy and by using the system of the conch. Jack, conversely, enters the scene with a dictatorial arrogance. He is controlling of his choir mates, and demonstrates great control of them.
Anarchy is rebelling completely against all forms of organized government. Although it seems Jack has a government of his own that his choir boys adhere to, it does not mesh with what Ralph has established before they come along.
The behavior of Jack is threatening to the existence of an organized group. Not an elected leader he comments:
“You’re talking too much,” said Jack Merridew. “Shut up, Fatty.” Laughter arose.
This is just plain rude. He found the second in command and squashed him with words as quickly as possible.
After Ralph was elected chief, many shades of Jack arose literally:
Even the choir applauded; and the freckles on Jack’s face disappeared under a blush of mortiﬁcation. He started up, then changed his mind and sat down again while the air rang.
Jack's "starting up" meant that he was about to get up and go earn his leadership by force, but he chose not to do that. He doesn't seem like the kind of guy who wants to be embarrassed and here he was... embarrassed. He wanted to save face, but didn't. Often people who demonstrate these behaviors will later follow through on them and that is what was so dangerous about the way Jack behaved during that first meeting.