Simple: when one of your BCC recipients happens to "reply to all" on the email, that person's identity is exposed to all the hidden recipients.
Dị mfe: mgbe otu n'ime ndị na-enweta BCC gị na-eme "ịzaghachi mmadụ niile" na email ahụ, onye ahụ na-ekpughere ndị niile natara ya.