Any disaster could lead to sudden decrease in population of an organism thereby decreasing its gene pool. Such disasters triggers genetic drift commonly termed as bottleneck effect.
Due to decrease in the population, the frequency of occurrence of any specific trait reduces as the frequency of alleles responsible for any certain trait also reduces.
Since habitat loss will cause reduction in both space and resources for giant pandas and hence a large number of pandas will die thereby reducing the gene pool and hence frequency of an allele.
