Corrosion of the body under the layer of paint may be one reason. If the metal surface was not cleaned or properly treated before the new layer of paint was applied, it is a matter of time before the corrosion manifests itself through the paint.

A more common cause would be poor workmanship. If the painting process was not carried out in a dirt-free environment (for example, a special painting booth), and the paint is not heat-cured, tiny bubbles could result later. These may not be very visible, but if you run your hand over the paint job, you might be able to make out the 'roughness'.