Clamps are a poor means of connection. Bad connections cause heat. However, that little panel shouldn't generate enough current to make much difference.
Did you install a fuse on the positive cable near the battery? It should have one to prevent this sort of thing.
Do you have a charge controller or did you just hook the panel straight to the battery? You really should have at least a basic charge controller to keep the battery from being over charged.
I'm going to guess a cheap solar panel that shorted out with no fuse or charge controller to protect it.