The inertia of the rocket
Since there is no friction in outer space, stopping a rocket's engine in motion does not stop it immediately. When the rocket's engine is stopped, the rocket continues to move forward according to the law of inertia. To stop the rocket, you have to get a reaction force by sending something forward in the direction it is going forward.
When the bus is stopped, my body momentarily turns backward. On the other hand, when the bus suddenly stops, my body is leaned forward. This is because all object tends to keep its moving state. This property is called 'inertia.'
Inertia is prominent when there is no interference with frictional forces. Outer space is a perfect inertial system in this regard.