Im guessing its because the panel is better protected if somebody crashes into it. The vertical would leave the plywood less exposed for a clean hit. Whereas a horizontal in the middle would leave open two large squares that would be weak if hit in the middle.
The book I read by JB corn only had a small square (cut off of a 2x4) in the very center, to make it so they didn't bow while stacked in storage.
Im sure others will say more but to me that seems why.