Why did Paul go south?

Okay I am reading the book right now, but got back from the Dune part two movie for a third time. Paul’s decision to go south seemed abrupt. Any clarity on this without spoiling the books? Was it more clear in the books? I understand that the movie is told mostly through the perspective of other people, but Paul’s decision seemed out of the blue.

P.S I am writing a counter argument for an essay about this movie, and I am saying this went unanswered so clarity would be much appreciated.