The tanker doesn't explode or crumple because the outside air pressure is balanced with the inside air pressure. The tanker crumples as the air is released out the it into another tanker because the air pressure inside the tanker has decreased this means that the pressure on the outside of the tanker is greater than the inside so the forces are no longer equal, this causes the outside pressure to push down on the tanker without a resisting force. As the pressure inside the tanker decreases the force being exerted per square meter on the outside of the tanker increases thus causing the tanker to crumple.