The most common place for pedestrians to die is on the side of low speed highways at night. They're typically struck from behind.
The other common mode is secondary to an original crash. Vehicles either are pushed into different roadways, over abutments, or down hills, which causes the vehicle to roll or otherwise crash into pedestrian areas without warning. This is most common in winter conditions.
I don't think deaths are the sole problem, and also I don't think nationwide statistics are appropriate considering that San Francisco (where Waymo's robotaxi service area is) has approximately zero low-speed highways.
The other common mode is secondary to an original crash. Vehicles either are pushed into different roadways, over abutments, or down hills, which causes the vehicle to roll or otherwise crash into pedestrian areas without warning. This is most common in winter conditions.