Hardy - Weinberg equilibrium states that the alleles of a gene in the population are always constant even after a generation. This equilibrium is affected by various processes like gene flow, genetic drift, natural selection, mutation etc.
In case of no mutation, genetic equilibrium of a population is maintained wisely, thus following the principle of Hardy-Weinberg equilibrium.
Natural selection, gene flow and genetic drift lead to the de-balancing of the allele frequency in a given population as it modifies the genetic makeup of the overall population.
Genetic drift refers to a fluctuation in gene/allele frequencies due to chance of events.
Gene flow: Migration of genes/alleles due to migration of few individuals in a population