I don't much care for the looks of the polished turd that wrote the article, but the allegations of plagiarizing stem from the following:
In Zakaria's column, titled "The Case for Gun Control," he began one paragraph with the sentences: "Adam Winkler, a professor of constitutional law at UCLA, documents the actual history in 'Gunfight: The Battle Over the Right to Bear Arms in America.' Guns were regulated in the U.S. from the earliest years of the Republic."
A corresponding passage in Lepore's New Yorker essay, titled "Battleground America," begins: "As Adam Winkler, a constitutional-law scholar at U.C.L.A., demonstrates in a remarkably nuanced new book, 'Gunfight: The Battle Over the Right to Bear Arms in America,' firearms have been regulated in the United States from the start."
Two sentences versus one in Lepore's work. Similar phrase structure, but different words all around.
If that's all he borrowed, it's a stretch to call it plagiarism.
But he's still a hoplophobic panty-wetting east coast sissy, so I don't have too much empathy for him.