It is still often considered politically correct to downplay cannibalism as a myth created by Europeans to justify imperialism and slavery. Cannibalism is a sensational subject and many accounts of it were exaggerated (see Arens The Man-eating Myth). But it has not infrequently occurred, particularly at times of acute shortage of food. Archaeology has now established the facts of cannibalism pretty conclusively (see White Prehistoric Cannibalism).
New Zealand is a good example of the factors which may have encouraged the development of cannibalism. The Maoris had arrived only about a thousand years ago, initially no doubt in quite small numbers. The islands, previously uninhabited by man, had no mammals except bats but were the home of a wide variety of flightless birds, some very large. Maori hunting gradually destroyed most of these, particularly the big moas. An explosion of human population hunting a diminishing supply of game would have led to a crisis of the same kind as I suggest had occurred in America about 10,000 years earlier. A period of intense tribal warfare and cannibalism ensued, unresolved at the time of European colonisation. An account of Maori warfare written in 1961 (before cannibalism became a non-subject) is at link.