After doing some research, I believe it is an over production and build up of sebum on the scalp. I have had acne problems, have rosacea, and my skin
and hair is very oily. I found balancing shampoos on line that are supposed to prevent sebum build up. Aveda is one of them, but I haven't tried it.
Sebum: substance with an oily consistency produced by the sebaceous glands. It lubricates hair, is an antifungal/antibacterial, and imparts waterproofing properties to the skin.