What caused this?

From what I can see the root cause could be a number of issues.

(1) Primers and top coats have different surface tensions. Did you mix the products? List the types of product and manufacturer of each layer. It's not good to intermix brands. Using the wrong primer such as a lacquer primer with a BC/CC system is a no -no.

(2)Primers vary widely as far as technology goes but all are moisture absorbent - moisture will become trapped in the primer and eventually migrate to the surface after a period of time. Lacquer primer is famous for this and in my professional opinion should never be used.

(3) Failing to allow enough tack time between coats or layers will cause this problem. Do you remember the wait times between layers?

(4) You didn't force dry (bake oven) the paint job I assume???

Answer these questions and I'll try my best to help you find the root cause(s).

sscuda