What causes pimples at the back?

It's called "bacne": Pimples on the back, aka bacne or folliculitis is usually the result of occlusion of oil glands and hair follicles with overgrowth of bacteria and yeast. The process is really the same as it is for acne on the face. It can usually be treated with topical washes of benzoyl peroxide or salicylic acid. Sometimes, oral antibiotics are needed to clear this up. However, it can be a chronic problem.