I airbrushed a cake in the shape of a sphere...the bottom half was styrofoam, the top half was cake.  The whole thing was covered in  a ganache shell, coated with piping gel, and then fondant.  I airbrushed the whole thing a few times, but noticed that the bottom part started buckling.  Any idea why?  The top looks great, but the bottom not so good.  Does too much paint cause this to happen to fondant?  I did spray more there than the top part.

I don't think the airbrushing is the problem.  The only thing I have ever seen over airbrushing cause is drips.  My guess would be gravity.

A^^ yep. Airbrushing doesn't cause a cake to buckle. It's a structural issue.

Or, your fondant was too soft.

AIm confused....the Styrofoam started buckling?

If I understand your description correctly, it sounds like the fondant on the styrofoam is degrading.

I have a couple of thoughts about that. 

I'd day that if you held the airbrush too close, or if your PSI was too high on the brush, because the styro is hard, the air pressure may have "pushed" on the fondant and made dimples.

If you know that you sprayed it more than the top, it could be an issue with too much moisture sitting on the surface.

It could be the piping gel degrading the fondant from the back, or keeping it soft. 

I never would use piping gel under fondant on a cake that would be eaten--only on a dummy.