It must've taken some time for the Executor to move out of the Astroid Field. If you look at Vader, His cape is on his back and not on his shoulders like it usually is. Which means He did something while his ship was being positioned. It would look out of place if you just cut to him just about to stand on the pedistal.
Sorry I haven't posted on here in a long time. I hope I can contribute more in the future.