I'd bet a lot of this behavior is heavily correlated with how we generally read top to bottom, which is in itself, probably an arbitrary decision made by ancient text writers.
For one, starting at the top and ending at the bottom is natural progress of things because of gravity.
I’m not sure if that means anything, but down-to-up seems very unnatural (of coure I can’t ignore my cultural biases). Is there any writing systems like that?
It’s all arbitrary.
is this a contrarian impulse or an anti-contrarian impulse?
A green flag for me that someone might be an expert is when their attitude towards answering a questions has that “it depends” energy.
Others have made a possibly more relevant point - in one direction, your arm/hand will block what you have already written.
I agree human mechanics is likely the reason people tend to write down rather than up though. But I’d say it’s more about our muscles, we’re stronger pulling our arms in than pushing them out. But I’m no expert so would never claim confidence in my assumption there.
Writing is done by people and people are almost always subject to gravity. It's one of the 4 fundamental forces. Energy minimization is not an arbitrary selection criteria, it's central to the fitness/design of all living things.
Any one can make arbitrary reasons to support a decision.
where is your writing-capable organ relative to your reading organs?