Why does this happen?

I'm certainly no expert, but i would imagine the Rust Oleum has anti corrosion additives in the paint that were not friendly to the Krylon base coat. You don't mention whether you decanted the rattle cans and airbrushed, or if you sprayed straight from the can. If you are coming right from the can I would say that the paint is too thick , as you can 't regulate it with the rattle can nozzle. Go get an airbrush, get some Tamiya,MM, or Gunze acrylic or enamel paint for your next try. You will be glad that you did.

Steve

I'm no expert either, but I suspect that Steve is right. I think that for whatever reason the Rust-Oleum reacted badly with the Krylon.

Out of curiosity did you sand it back down? I'm wondering if it was just the paint that reacted or whether it actually went all the way down and reacted with the plastic itself. Some paints are just not designed for plastic.

I sprayed it right from the can,and then I sanded it back down.I think it was because there was 2 different types of paints because no matter what knds of paints I use it happens anyways..