Because you sit down on your buttocks and place a lot of pressure on them, they could become over heated and the area might perspire a lot. This creates a warm and humid surroundings that is a prime breeding spot for many different kinds of microorganisms, which includes those that can cause pimples on your buttocks. For those who sit down a lot at your job, it can cause this factor to be accentuated and if you are overweight it will add to the problem. If your immune system is compromised owing to sickness or medication, you might be more prone to these types of pimple infections.