I took Anthropology 101 at Auckland Uni and was lectured by Doug Sutton. He suggested the reason they sailed against the prevailing currents is that when they

I took Anthropology 101 at Auckland Uni and was lectured by Doug Sutton. He suggested the reason they sailed against the prevailing currents is that when they ran out of food and water without finding land, they let the currents carry them back to their origin. The other way around is certain death. Also that they may have been driven by overpopulation pressures to seek out new land.