Hi Midnight ~
Yes, I
have noticed that physical intensity, but I like it

. It helps 'to take me where he's at' in the song, without being distracted by 'him.' That may be part of why he does it, in fact. I don't really think so, but it's a possibility. I think he's doing, as he does so well; going deep inside and bringing the song forth from that realm. It's what we hear and love so much.
Diluting delivery of the inside world, with/by attention to the outside world, is too likely what would happen. I notice that when many/most[?]singers deliver their most intense lyrics, they tend to close their eyes and either raise their head to the 'sky,' or bend their head down [both being 'go within'/'go above' postures, i.e. away from 'the world,'] as they go, and take us, to the lyric.
It reminds me of his comments, I believe, in 'How To Read Poetry;" something to the effect of step aside, get out of the way, and let the poetry speak for itself. What he does is a very effective way of 'getting out of the way.' There's something extremely appealing about seeing someone engrossed in what they're doing, seeing that deep level of concentration. Oblivious to the world around them. It's very attractive to me. Sexy, in fact.
The best photos of people are so often where they're unaware of the camera, and fully engaged in something. It's that same kind of thing.
~ Lizzy