To answer the first part of your question: we don't know-- at least we don't always know. Sometimes it was because of population/resource pressure, though this is not always the case as most islands were not near their carrying capacities before new islands were settled. Some historians have speculated about rivalries or wars forcing people into exile, again we know it happened sometimes but certainly wasn't always the case as island populations usually maintained strong links for generations. Possibly it was secondary bloodlines who by settling a new island could be the highest bloodline-- or secondary bloodlines who had gained too much mana or spiritual authority and threatened to upend society and needed to move away; though again not always as island societies often had other ways of dealing with these events like marriages to other districts or with high lineages on other islands. Or perhaps, it was just an adventurous spirit... exploration a sort of past time for navigators, though even then, considering the resource cost of long distance exploration, it wouldn't have been the sort of hobby you engage it very often. So really, we don't know, likely migration occurred for different reasons at different times, there are a number of factors that could have influenced large settlement events.

As to the second part, how did the know where to go, to put it simply, they had already found the islands and knew where they were before anyone began migration (except in a few cases). Generally, the settlement of islands in the Pacific took generations. Islands in the Pacific were NOT habitable or hospitable places. There are virtually no endemic edible plants amongst the Pacific Islands, with the existence of endemic edible plants becoming less likely the further east we move (therefor the more isolated the islands with lower rates of biodiversity, but higher rates of endemism). Islands like Hawaii had only a couple of edible fern species-- all the other foods we associate with the fertility or fecundity of Pacific Islands comes from the cargo crops of the settlers (coconut, sugarcane, mountain apple, taro, banana(s), breadfruit, pig, dog, chicken, etc, etc..) or from the past couple hundred years of colonialism-- many of the tropical fruits we associate with the Pacific come from South America. Because the islands did not provide a bounty upon arrival (aside from large flightless birds like those found, but now extinct, on Hawaii or Aotearoa (new zealand)), the islands had to be terraformed, or transformed before large scale settlement could begin. This could take time-- generations in the case of resource or soil poor coral atolls and maybe a generation in the case of larger islands like Hawaii or Tahiti. Small founding populations or temporary settlements would over time give way to much larger settling populations.

Before the islands could be settled, they had to be found, this was usually accomplished by navigators who it seems went out looking beyond the horizon to see what they could find. We don't really always know the reasons why. We do have some understanding of the general methodology of how navigators went about finding islands. One of there most important strategies was to begin exploration by settling out and sailing against the wind-- a strategy the can seem funny at first till we ask ourselves why this might be a good idea-- and the easy answer is; if anything goes wrong, you can turn around and use the wind to get home much quicker. Otherwise, navigators would use natural signs to know whether or not they were near land-- the presence of birds, flotsam, changes in wave patterns, cloud shadows (islands, especially high islands, are visible from very far away) all provide clues that land is near-- if not giving away its location exactly. Once land is found, its simply a matter of reading the stars and getting a coordinate in order to come back.