Drywall is generally recommended to be hung vertically for two reasons. First off, the seams are less noticable. If you hold a light at a low angle to a flat surface, you are much more likely to see the imperfections. Picture walking down a hall… If the sheetrock is mounted vertically, your eye will notice the seams up ahead more noticably because of the acute angle. If it is mounted horizontally, the sheet rock seam is a slightly more obtuse angle (unless you are 12 feet tall) and slightly less noticable. I think of equal importance is the ease of finishing the joint. On vertical mounted sheetrock, the joint runs from ceiling to floor and it requires that you stretch and stoop in order to finish the joint. On horizontal, the joint is an easy to reach and consistent 4 feet off the floor.
As for why it is mounted top down… You put in the ceiling first. Then butt the walls up to the ceiling. This helps lock the ceiling into place and keep the joint from cracking.
All that said… I am not a professional, but I have done LOTS of sheetrock work and it is as good as any I’ve seen. I always mount it vertical. For a single man operation, it is too hard to hold an entire sheet four feet off the floor and attach it to the wall. It is much easier to cut the board to the exact length then stand it up on the floor and attach. BTW, if you are doing this and not just asking out of curiosity… use screws, not nails. Buy a screwgun if you need to. You will be much happier with the results