Why might someone suddenly have cystic pimples?

Don't ask why: If this is a young person, don't look for an explanation for cystic acne. There's nothing you can change about hygiene, sex, lifestyle, or diet that will make it go away. You will probably need isotretinoin. If you're an adult, it may be a warning of one of the unusual glandular diseases. Usually it never gets explained. Manage it with strong meds. Don't try to solve it non-medically.