Air carrier Ground Personnel reported a ground service van driving behind an aircraft during push back. Push back was stopped.

Narrative

Flight ABCD/Day 0 was being push from the gate at approximate time departure. Marshaller signaled wing walkers breaks release and began pushing the aircraft when a van drove behind the plane even when wing walkers are present and in position. Engine one was running as this was an air start. The signal of 'engine on' was being performed by the wing walker. Several vehicles are at still position indicating and acknowledging they can see the wing walkers are giving signals to not penetrate the area behind the aircraft. Only van drove behind the plane causing the reaction of the push back driver to stop the push immediately. I visually followed the van and waited for the van to stop at a location to keep it on sight and not lose track of the vehicle. Once I noticed the vehicle had completely stop I drove to the location and made contact with the driver. I introduced myself and advised him he had driven behind a live flight when the aircraft was being pushed from Gate XX. The driver acknowledged and apologized and said sorry for what had happened and noted his been doing this work for a long time and this was his first incident of such occurrence. I noticed the driver was feeling very disappointed about what had occurred and told him to please pay more attention when driving on the ramp and to pay more attention and to identify the presence of agents when signaling with the wands.