Why does Claudio think Hero is cheating on him in Much Ado About Nothing?

In Much Ado About Nothing, Don Pedro's half-brother, Don John, hates both Claudio and his brother. He devises an evil scheme to make Claudio and the prince doubt Hero's purity and faithfulness. He hires evil Borachio to court Margaret, Hero's maid, dressed in Hero's clothing and appearing at her window. He tells Claudio and the prince that they can witness Hero's infidelity that night. They do so, and Claudio is incensed.