I know nothing about the incident nor about that particular person (except his most famous short story), but what I can see on this video is an awkward and misplaced move that sometime happen in nervous situations to some people. Like someone was trying to pat other person's shoulder and shake their hand (using his left) at the same time while speaking and failing both. If you ever had similar situation, you know what I mean.

Please, mind it, I don't say that's what happened, I just say it looks like it and may be interpreted this way. A person can hide his intentions using awkward behaviour, of course. Prior information and context tip the scale in the end.