It's not clear what finally pushed things over the edge into active war - digital records get unreliable the closer you get to the Fall, and the people who survived and know about what went down and the people who are talking are two separate sets.

As far as historians can tell, the stresses of limited resources, resource disparity, old-school memetic tensions - ethnic, religious, that sort of thing - and whatever else finally hit a point of no return. Then bootstrap-capable AIs were deployed as weapons, and things really went to hell.