Bottleneck Effect
Genetic drift is particularly strong in small population of say, 100 breeding pairs or fewer. One such situation that could lead to this phenomenon is the bottleneck effect, where the population is small enough to facilitate drift.
One example of genetic drift in a population bottleneck is in South African cheetahs. The cheetah population was severely reduced during the ice age and hunted to nearly extinction in the past, such that they are homozygous at over 50 different loci and have less genetic variation than inbred laboratory mice.