It couldn't have killed them off or they wouldn't have been around to die out again in 10,000.
My suspicion is that it is a mistake to look for "the one big reason." More likely a cascade of events including climate change ( a repeating variable,) human hunting ( a new variable, perhaps) and even Firestone's asteroid/comet ( a one-time variable) coming together. Start introducing new variables onto a population already under stress from climate change and anything is possible.
Something is wrong here. War, disease, death, destruction, hunger, filth, poverty, torture, crime, corruption, and the Ice Capades. Something is definitely wrong. This is not good work. If this is the best God can do, I am not impressed.
-- George Carlin