In Godzilla (2014 Film) why is Godzilla hunting the MUTOs in the first place?
According to Wikipedia (which itself cites the book Godzilla: Art of Destruction):
David Callaham's first draft featured early versions of the MUTOs where they were established as ancient enemies of Godzilla but never established as to why. When director Edwards came aboard the project, he created a back story to bridge the gap between Godzilla's connection with the MUTOs. Edwards brainstormed the idea that, "When these Godzillas were on Earth, there was another creature that would kill them and lay its eggs inside their dead bodies. Therefore, if these creatures ever came back, part of their life cycle would be the ability to attract Godzillas to the surface to kill them for reproduction."