I remember I read somewhere about the fact that right-to-left script was preferred when carving script on stone. as right hand people supposedly prefer to hold the hammer on their right hand.

I'm not sure if that's really the cause. Furthermore in many ancient scripts the direction was quite unimportant, in some cases even reverting in each line (Boustrophedon).

Interesting take on direction of writing: http://shkrobius.livejournal.com/292987.html