Perhaps he discovers a new power source or material in his adventures that can be used to make bombs. And he realizes that bombs could be a useful addition to his arsenal to take on stronger foes. Also, he might be inspired by other characters who use bombs in the fanfiction world he exists in.
In fanfiction, Naruto could be making bombs to protect his village in a more extreme way. He might have seen the destruction caused by powerful enemies and thought that bombs could be a way to create a deterrent. Another possibility is that he is tricked or forced into making bombs by an evil force. But as he makes them, he could find a way to turn the situation around and use the bombs for good instead.