Whatever the explanation is, it better not have something to do with Anakin discovering how to do it for the first time. In Episode IV, Obi-Wan says to Vader, "If you strike me down, I shall become more powerful than you can possibly imagine." I doubt he was preaching to the choir. And it clearly seems Vader is surprised once Obi-Wan vanishes. (Otherwise, why is he stepping on Obi-Wan's cloak trying to figure out what happened???)