That is the natural convection of you oven. It seams a little slow turning but you can clearly see your cold air coming in, heating up and circulating around the chamber to exit at the top.
Fire to the left creates clockwise circulation and vice versa.
You can better see it due to the lack of oxygen creating black smoke. The natural convection also occurs when there is no fire and you can see it when there is steam in the oven (when baking bread).
I guess the raised deck slows down the circulation of the air?