The correct answer is - mass extinction.
The most dramatic extinction of large prehistoric mammals occurred when the ice age was ending. The climate suddenly changed dramatically, thus the environment changed very quickly in response. Also, there's indications that maybe a meteorite hit Greenland and caused dramatic changes in just few hours and days. The larger mammals were all very specialized animals, so even small changes in the environment are fatal for them, as seen in this case.
It is interesting though that only the megafauna died out, while the medium sized and smaller animals survived, largely because they are not very specialized but are much more opportunistic in their food choices. Some of the large mammals that died out in this period are the smilodon, homotherium, american lion, short-faced bear, mastodon, mammoth, American horses, American camels, giant sloth etc.