I think it could be dyskinesia. There have been a few examples now where he has had trouble with handshakes, especially when he is surrounded by a lot of movement and noise. Like the rising ovation for Gorsuch or the photographers here. He also reportedly held May's hand because he wasn't comfortable walking on stairs without support.
There was also this time that
he had trouble shaking hands with Thiel and this occasion where
he evidently could not see Giuliani even though he was sitting directly across from him.
When you start looking for it, you can actually start to see a troubling number of times where Trump doesn't seem to be fully in control.