Watching the video, it looked like the booster was still going at separation.
I've made that mistake many times on kerbal.
I've made that mistake many times on kerbal.
To expand, this has to do with keeping liquid propellantws settled. If you stop thrusting your propellant will start floating around in zero g. That means you need to thrust to settle it back down before you can fully engage your engines. That complexity is sidestepped by just keeping the engines running at the rocket equivalent of idle.