It sounds like the paint you used caused a reaction with the original paint. You might have sprayed too heavy, or the temperture didn't make the paint dry quickly enough. There are reducers in spray paint, that evaporate after they are exposed to the air. It's a paint thinner pretty much.. Once all the paint thinner evaporates, the paint is dry. If you sprayed the paint on too thick, the paint thinners take longer to evaporate, and will start "thinning" the original paint, which causes the original paint soften, which will cause "cracking". It'll look like a spider web sometimes.
Also, you said you primed it. If you didn't wait long enough before painting over the primer, basically the same thing will happen, because the primer isn't "set" or "cured". It reacts to the thinning chemical, and softens just the same. That's probably what happened in your case. That's why it's extremely important to spray several thin layers, instead of a few thick layers. If that thinner doesn't evaporate fast enough, you'll get a big mess, and have to start all over.
