Based on the documentary Moana, maybe there came a time where things were not going well at their current place so they set out looking for a new place.

Or maybe warring peoples invaded from the west, so they picked up and headed east looking for a new home.

Or maybe there was overpopulation on their island so they looked for a new place to spread out.

Or maybe they were just like us and some young guys were thrill seekers/adrenaline junkies/adventurers/etc. and they just wanted to head out to see what they could find.