Sorry for bumping this thread.
It is surprising to me that no one has mentioned what I learned (who knows where?) in my childhood as one reason for writing direction reversal.
What I remember from way, way, way back is:
Especially with the advent of ink and paper/parchment/papyrus flat writing surfaces, the problem of a right-handed person writing RTL is that the writing hand can easily smudge the freshly written text as the writer moves to the left. Switching to LTR obviates that problem. A right-handed person sometimes, when writing Hebrew, e.g., contorts the right-wrist a bit so that the hand remains below the baseline, avoiding the contact that could lead to smudging.
If the custom was TTB-RTL, the smudging problem could occur. Simply rotating the surface 90 degrees counter-clockwise changes the same text to LTR-TTB, eliminating the smudging.
I have seen some people, left-handed mainly, contort their writing hand to hold the pen/pencil above the baseline to avoid the same-line smudging problem (hoooray for quick-drying ball-point and felt-tip pens!), and I have also seen some left-handed people turn the paper 90 degrees to the right so that for them the direction is TTB-RTL, again avoiding smudging.
Having never written on clay, I don't know if smudging the just-written text was a problem with that medium. It seems like it could have been, as the entire tablet had to be moist throughout the operation (maybe not had to be, but I don't know if you could continue impressing line 14 if the clay of adjacent line 13 is already dry enough to prevent accidental erasure).
Did cuneiform scribes suffer from carpal tunnel syndrome or other forms of tendonitis?
A future topic (may have already been discussed): what determines handedness? I feel for the left-handed in our right-handed world. The abuse they still to this day suffer for a condition that is surely not of their choosing is awful.
(Avatar is an electric motor consisting of a bit of wire, a couple of paper clips,
two neodymium magnets, and a pair of AA batteries. A very cute demo of
minimal technology, and likewise completely useless for any practical purpose.)