Brother the additional gas IS the external force. I'm not suggesting air resistance is slowing it down, I'm saying it's literally snowballing
Man… idek anymore. Look. At the start the shockwave was only moving this small amount of gas and had all of its energy concentrated on this small spot.
Then here, the energy is spread on a surface like 50x bigger and it's pushing a LOT more gas as a result.
This means that per m³ there is way less energy and it's pushing way more gas than at the start.
Since there's now more gas and less energy to push it, the speed ay which the gas moves will be much slower. The gas is also moving towards even more gas which will snowball onto the pile, spreading the energy even more and making the mass that's being pushed even higher.