For quite a while the consensus theory was that this extinction was caused by human hunters.
I used to be quite resistant to this notion, as it just seem unlikely to me that human stone age hunters could exterminate so many animal across an entire continent in just a few centuries.
However, we have good the same thing happened in Australia, New Zealand, Madagascar and other islands. The mega-fauna disappeared within a couple of centuries of humans showing up.
So I'm a somewhat reluctant convert.
Why didn’t the mega fauna of Asia and Africa die out as well? I would have loved to see that huge armadillo cousin Glyptodont with its mace like tail walking near a stream during the last ice age.
Let’s face it, the Indians did not have repeating rifles like the European invaders of Australia, New Zealand and Madagascar. Also I do not have the impression that the Clovis population was very numerous.