I'm pretty sure war-prisoners, criminals, people who have broken taboos, etc. etc. had a priority, but punishment was not the main motive.
There was just this sadist kick that Fijians got out of cannibalism that is difficult to understand in today's context.
They would even casually chew on raw pieces of human skin or human ears as chewing tobacco. :^)
Festivals always were good reasons to eat lots of people. Whenever a canoe was built there was one.
Although, the festival about a chieftain's canoe being finished was infinitely more spectacular and gruesome than when a regular guy's canoe was finished.
According to traveler Alfred St. Johnston a man had to be slain for the laying of its keel, a fresh man was killed for every new timber that was added, more dead men were used as rollers to aid its passage to the sea, and then fresh men were killed to wash the canoe with blood. :^)
I think Fijians were somewhat on the same level as Aztecs when it comes to gruesomeness, although the Aztecs seemed to hold more of a religious motive behind it. (^:
It somewhat reminds me of the stuff that went on in Auschwitz and Soviet Gulags personally. :^)