Population size fluctuates. Subdivided population structure. Successive generations are a random sample from the parental gene pool. That is, indeed, the Hardy-Weinberg expectation. Understand that random samples are NOT selection.
Drift occurs gradually over many generations.
The founder effect results from a single small sample when a population is first founded. Inbreeding can reduce the effective size of a population. The frequency of alleles with the same fitness will change at random through time in a process called genetic drift. A small founder population may have a nonrepresentative sample of the ancestral population's genes. This founder effect is the result of a single small random sample.