Re: What caused this?
could have been a few things
1 too dry a spray technique
2 humidity trapped in your spray , yes it can get in there just by moving through the air
3 paint flashed off too quickly trapping solvent beneath
4 your top coat could have softened your primer an brought its pigments to the surface a common problem for example: when trying to spray light colors over reds or dark primers but rarely seen when using light color prime an dark color, but still possible .
hope that helps explain it