Keloid formed over bacterial acne, which likely would have cleared up on its own.
Keloidal scarring is sometimes idiopathic (with no known cause), but most of the time it’s due to contamination of the wound during the healing process. If an infection doesn’t develop, but the contamination is still present (like, say, if you worked in a coal mine, had dust over a particularly bad pimple, and rubbed it into the comedo accidentally), the body produces an excess of hypertrophied scar tissue, and creates a keloid. Unlike the acne pustules, keloids will not go away without surgical removal.
An Introduction to Dermatology. Norman Walker, 1911.