Explain why disagreement with Zionism is Anti-Semitic

Zionism is a broad label. It began as a secular movement, mostly backed by European socialists who were, obviously, of Jewish descent. The spiritual aspect came later, and even later, the fundamentalist aspect.

I would recommend looking into the history. There are plenty of resources.

Let me answer this way - disagreeing with some variation of Zionism does not necessarily make someone an anti-Semite. The history of disagreement within the Zionist community should be enough evidence to support this claim.

But as with any discussion that revolves around religion, or someone's nationality, and especially a discussion that revolves around someone's religion and nationality, we should extremely careful with our language - careful not to oversimplify. Careful to understand first and make judgments second, if we are to judge at all.