How does cunnilingus cause hiv?

No particular sexual practice is the cause of HIV infection. HIV is a virus that usually is transmitted by sex, but by far the most common sexual practices that transmit the virus are vaginal and anal intercourse. Oral sex rarely transmits it, and cunnilingus (oral-vaginal contact) has never been known as the way it was caught. That doesn't mean cunnilingus is zero risk, but it's very low.