we don't know the reasons the Polynesians expanded. It is highly doubtful a lone couple of polynesians set sail on the high seas to find new islands. The amount of provisioning and boat building...analogous to modern government sponsorship
What's even more likely is that tribes who lost a war were tied into their own boats with meager rations and set to drift at sea. Most probably died, but a few got lucky. Perhaps this was part of a ritual.