Asteroid, super volcano, ecological collapse, nuclear war (with its knock on environmental effects), pandemic could all cause human extinction.
edit: Also, human extinction shouldn't be the bar. Prosperous humane existence for all, or hope of someday achieving that should be the bar.
Nuclear war thankfully has still not occurred. Even if it were to occur one can hope that we would have enough sense to limit how many weapons were used. The fact that MAD works wherever it is in place indicates that humans are not as gung ho on self-immolation as the media makes us out to be.
Ecological collapse and climate change are two different things. Total ecological collapse is much harder to accomplish barring a massive event such as an asteroid impact.
WRT ecological collapse, aren't we experiencing the front end of a mass extinction in the oceans? Can humans survive such an event or will it be a domino effect up the food chain?
I don't usually like ruminating on this stuff but it's worth considering our future without AI if we are going to consider what it may be like with it.