AIUI, it was the irregularity of the uncontrolled intersections combining with the “novel” (from the POV of the software) driving style of the humans. In dense areas during outages signaled intersections don’t actually degrade to 4 way stops, drivers act pretty poorly.

The normal order and flow of traffic broke down. The software determined it was now outside its safe parameters and halted.

Certainly not ideal, and the should be a very strong regulatory response (the gov should have shut them down), and meaningful financial penalties (at least for repeat incidents).