The answer to this can be found on the second page of Chapter 5. There, we are told,
"But he had to know, he had to know..."
Mafatu has been fascinated in a horrible way by the Sacred Place and by the idea of the cannibals coming. He defied their god to get the spearhead. It was the fear of the cannibals that drove him to build the canoe so he could get home. Although Sperry does not say it explicitly, it seems that this is one of the ways that Mafatu has to prove to himself that he is brave. He has to face his fears and actually see the cannibals, not just run away without knowing. This is why he goes to the Sacred Place (or at least to where he can look down at it) when he hears the drums.