There just doesn't seem to be a pattern to the extinctions at all - camels and horses (in N. America), but not deer or elk; short-faced bears, but not grizzly bears; dire wolves, but not grey wolves. The "mega-fauna" were wiped out, but the run-of-the-mill fauna weren't.
I think the problem is, you might be looking for a “big pattern” that explains them all, while each separate extinction probably has its own unique causes. Humans cause some extinctions, other predators cause some, changes in habitat, climate, or availability of food sources cause others, etc.
That's a very interesting point. It was animals that weighed over 200-250 pounds that died off mostly. Since very large critters require more food, some sudden decline in food sources makes sense to me as the cause for the big animal die-off. I've never bought into the idea that human hunters caused the megafauna extinctions. They may have picnicked on already-dead mammoths and even killed one now and then but they were much more likely to go after smaller and less dangerous prey given their primitive weapons. In general this new theory makes sense to me.