Robert Nozick was not a libertarian. He has never claimed that he was. In the Preface to his 1974 book Anarchy, State, and Utopia, he is lukewarm about libertarianism. He writes: “With reluctance, I found myself becoming convinced of (as they are now often called) libertarian views, due to various considerations and arguments.”
He has based his arguments on the claims about rights (mainly property rights), but his treatment of rights is weak. He starts with a situation in which people are living in a so-called “state of nature,” which is a mythical concept. In his later work, he has admitted that he had developed some doubts about earlier view on rights. One of his intentions in writing the book was to refute the libertarian thinker Murray Rothbard, but Nozick never took his argument with the libertarians forward. After the publication of Anarchy, State, and Utopia, he dissociated himself from the book and did not respond to the criticisms of it. His writing work after 1974 is unrelated to libertarianism.
Anarchy, State, and Utopia became influential in libertarian circles, because Nozick was a high profile professor of philosophy and because some libertarians saw him as the libertarian answer to John Rawls.