The bottleneck event that decimated the bison was caused by

The answer is an overhunting.

The bottleneck event means drastic reduction of the population size because of environment changes. The consequence of this is the reduction of genetic variation. The bison population is a good example of the population bottleneck. The bison population drastically reduced its size due to overhunting which culminated at the end of the 19th century, when bisons almost extinct.