There wasn't any concrete reason why Godzilla was fighting Donkey Kong; perhaps the gorilla who munched away on bananas paved a certain appearance to a particular king of the jungle that he didn't like: regardless the mutant dinosaur fired his nuclear blast at the ape. DK of course returned the favor after being burned by punching Godzilla in the face; they kept tearing throughout the jungle but 1 thing was for certain and that was even though Donkey was outmatched he wasn't going to give up a fight. "Come on ya big lizard; I've took on many Kritters in my time and you're just 1 big glorified Kremling when it comes to it!"