Occam's Razor applies. The simplist answer is human predation. And it's been proved; there is an example. Think Dodo.
Of course when it comes to the human mind, in practice, Occam's Razor is often powerless against the combined forces of human self-interest and self-deception. Both the politically correct leftists and some "conservatives" have strong political motivations for denying that ancient mankind could have exterminated entire species.