1-an "unlucky" individual in a population does not reproduce, meaning some of their genes are not as frequently observed in the population's "gene pool."
2- a disaster of some sort wipes out a group of individuals from a given population, reducing genetic diversity because only a subset of the original genes of the population's "gene pool" are left after the disaster.
3-an extreme wind event may move a species, say flying squirrels for example, from one habitat to another. The individuals who are blown to the new site have only a certain subset of genes from the original population's gene pool, so the new population has a different genetic structure/gene frequency distribution.
4-when a population is completely obliterated locally, meaning no individuals are left. However, some individuals of the species may exist in other habitats.