I'm pretty sure it's this simple: The Japanese, when writing horizontally, do it just like us, from top left to bottom right. But, unlike us, they can also write vertically. And when writing vertically, the orientation becomes top right to bottom left. And that's how they do it in manga, just like in other regular japanese books.
FappingFury wrote...
I recall in digital design class that the human eye instinctively reads top left to bottom right
I find that a little dubious. Isn't it an instinct
just for us, who are used to solely read from left to right, top to bottom?