I think it is basically butterfat globules which have come together. Butter is formed when the sheath covering fat globules breaks, and then the fat molecules inside join up with one another. There is always some agitation during the processing of milk (milking, storing, transporting, bottling, etc) that very slightly can break up the fat globules, essentially "churning" a small amount of butter, which can rise and float on the surface of milk. The fattier the milk, the more likely this is to happen, and also the less fat that can get retained in the curd and instead remains in the whey, again rising to the surface.