Just been watching this which also has a T-Shaped door (and led me to this thread).
My thought are that a T-shaped door is the minimal size. If you didn't have a door but it was cold at night you'd hang a 'curtain' in front of it. The smaller the door, the better and the T-shape is arguably the minimum practical. If my theory were true I'd also expect a step at the bottom and a lintel so that the curtain could seal on all 4 sides.
