Why do allele frequencys in a gene pool change in gentetic drift?

Let us take a random example, which is genetic drift.

A small population of beetles are on a small island. Some few are green, recessive, (gg) and most are brown, dominant (Bg and BB). So you see that the majority of the population are brown, which also happens to be the adaptively favored color. Bird populations go through a boom on this island and almost all the green beetles are eaten. The allele frequency will change through this random process of genetic drift.