Techneut: Since a grand is transported on its side, this could be because the soundboard should be transported vertical, not horizontal. That could be a reason that an upright is transported the same way, that th soundboard is horizontal (so not on its back). So probably Mergaronin is right, that all pianos should be transported with vertical soundboard, regardless upright or grand.

Interesting theory. It makes me worried though about concert grands, that get wheeled about on and off the podium, and whose soundboards spend all their life in the wrong position....

I would rather think a grand is transported on its side a) to be able to fit it in the removal van and b) to get it thru the door.