hes left handed. for some reason lefties write like that.
My boss said it was to avoid dragging the edge of his hand over ink before it dried. Writing right handed sweeps the edge of the hand away from wet ink and left handed can drag over wet ink unless you curl your hand up like that. I also wondered if Arabic is written top left to lower left down the page - which would also mean you might curl your hand up to avoid dragging through ink but I am not sure - never having tried!