Pimples on the buttocks are commonly caused by clogged pores due to sweat, dead skin cells, and dirt getting trapped, especially because the area is often covered and subjected to friction from tight clothing. Other frequent causes include skin conditions like folliculitis (inflammation of the hair follicles), hormonal changes, poor hygiene, prolonged sitting pressure, certain diets, hair removal irritation, genetics, and stress. The pimples may not be true acne but related skin issues triggered by these factors.