I'm sure e.e.norcod will be checking in momentarily to give the definitive answer, but I believe those bumps are caused by the same thing that causes goose bumps (goose flesh). Tiny muscles under the skin tighten and cause hair follicles to become perpendicular to the skin's surface. The skin around the mouth of the follicle puckers and makes little bumps. If there's hair in the follicles, it tries to stand on end.

The evolutionary advantages of this are at least two-fold: If you are an ape and your hair stands on end, you look larger, and therefore more dangerous to predators or rivals, and; if your hair stands on end, it traps air within your pelt and insulates you better from cold than if it lays flat.

I assume that since one of the triggers of milk ejaculation (expression) is the contraction of tiny muscles under the skin of the breast sac (and around the alveolae), some of them must be the same muscles that cause goose bumps.