What caused the Pleistocene Mass Extinction?

[brotherbear]

I have spent some time pondering why the black bear and the grizzly survived in North America when the larger short-faced bears perished. i'm thinking that the surviving species might have had a wider range of food options. Were the short-faced bears diggers and fishers? Also, and perhaps the surviving bears' greatest advantage is the ability to sleep during long months of hard times. 
I also believe that the grizzly's size adaptability is a great survival trait. Those living in harsh terrain are small in size, thus in need of less food. Few large mammals of a singular species can range in size from -300 pounds to 1500+ pounds. The puma ( cougar ) was perhaps able to sustain on rabbits and rodents while the big cats had a much more difficult time. 
I believe that the culprit of the extinction was weather related; affecting the vegetation which in turn affects the animal kingdom. Reply