I'm not sure, but nearly every car I've ever seen that's been parked around horses has horse-bites. My guess, which is admittedly not a very well-informed one, is that car paint tends to be shiny which can make a horse interested enough to investigate it.
My dad's in town so I actually got to ask him this-- he says that clear coat can flake off if it was damaged by UV rays or if it didn't bond correctly to the paint for whatever reason.