Air carrier Captain reported a critical ground conflict during push back when a ground vehicle drove behind the wing walkers. The push back tug driver had to brake suddenly to avoid a collision and in the process the towbar pin was sheared.

Narrative

We had just received clearance to push back into the alley from gate in ZZZ. I released the brakes; and informed the tug driver we were cleared to push. He read back the confirmation. We had not moved 5 ft. when we felt a sudden jolt and the aircraft stopped moving. I queried the tug driver about what happened; and he informed us that a ground vehicle had continued driving past the wing walkers behind the aircraft and that he had to slam on the brakes to avoid a collision. Due to less than ideal audio caused by an intermittent headset; I asked the tug driver twice to confirm that the aircraft did not impact any vehicle or equipment; and he confirmed twice. However; he stated that the towbar pin appeared to have been sheared due to the force of the abrupt stop. I set the aircraft parking brake and informed him of such. We then coordinated with hub maintenance for an inspection of the nose gear (which was signed off with no damage noted) and the ground crew retrieved a new towbar pin. The rest of the push and flight were uneventful.