Question: I just realized that the directions in "UPload" and "DOWNload" seem arbitrary to me as a non-native English speaker. I took a look at a couple of dictionaries and they said that this word is a result of merging "down" and "load", which doesn't seem to explain anything. Where could those two directions come from?

Answer: 
OED noticed the word for the first time in 1976, it seems.

1976 was very early in the development of computers. FTP (File Transfer Protocol) was designed 1971 and TCP/ IP followed in 1973. These two protocols made any transfer of data possible for the first time.

Servers or internet wasn't something anyone could imagine yet. Instead scientist were still hammering out the basic how of a data transfer.

In these years, several conceptual models for the transfer of data were developed. Most used and known nowadays is the OSI model with 7 layers - and these layers have an up / down direction.

Now, if I want to get data from another computer, I technically send the other computer a request, which then converts said data "down" to the physical layer, until all is left is a physical signal. This signal is then send to my computer, and I convert it again (this time "upwards").

This also is a likely explanation why "upload" appeared years later, in 1980 according to OED. Upload was coined as the simple opposite of download.

Though, while this is what I first thought of, it might be that there is also a hierarchy meaning in play as well.
Looking at the OED first use, "1976 [Science 7 May 518/1](http://science.sciencemag.org/content/192/4239/511) Software at any level can be developed on a host minicomputer and ‘down-loaded’ without code conversion", it appears to be talking about loading code from a host to a microprocessor. No real help there but I wonder if the author was thinking of taking reference books down in a library to copy out texts and then putting them back up on the shelves?