Some chemical reaction between the paint and the clear coat?  Otherwise, no idea.

This probably doesn't help you, but I use Krylon's Chrome paint to replace the kit chrome on some kits.  For example, I have the old Monogram Tom Daniel's Red Baron hot rod.  I stripped the chromed parts, so I could clean up sprue gates, seams, etc.  I used the Krylon Chrome to paint them.  It has a more "realistic" tone to it, more like real chromed parts, than the bright chrome used to coat the kit parts.