its called micro blister. it can be caused by a whole heap of things. its a bubble in the top coat. can be caused by improper surface cleaning. dirt on the surface to be sprayed can trap moisture that blisters when temperature or presure increase. also use of a quick dry thinners especially if sprayed at too high a pressure. excessive film thickness, or insufficient drying time between coats can trap solvent. as can too heavy an application of under coat. and last but not least, contaminated compressed air. its a mine field this spraying game.
if the damage is only in small patches you can flat it back and re top coat. if its bad you will have to strip the paint off and start again