Andrew S

I'd take a risk and say that most budo etiquette was influenced in general by Zen teachings (popularized by the bushi caste) and particularly the Ogasawara Ryu, which taught bujutsu and tea ceremony, among other things. Seiza and therefore zarei were heavily influenced by this school.
Don't forget the Edo-period training for a bushi to be constantly alert (probably an imagined need rather than a real one) which also manifests itself in reiho (which hand do you put out first in zarei?)
Finally, the samurai were the only caste with enough free time to come up with complex mannerisms.
So, in a sense, much of aikido etiquette stems from samurai etiquette.