The writing direction in Arabic, Hebrew and other Semitic writing systems has been inherited from the Egyptian.

So any hypothesis about the origin of the right-to-left direction must discuss only how the tools were used for the Egyptian hieratic (written with brushes) and hieroglyphic (carved with chisels) writing systems.

Maybe it was easier to carve hieroglyphs in that direction, but I am not aware of anyone testing this hypothesis in practice.