Single authors referring to themselves as "we" is still commonplace today, and already Newton was we-ing in Principia. There is even a Latin term for we-ing, nosism, from "nos", which is Latin for "we". Googling "royal we in scientific writing" returns surprisingly many hits, including a Wikipedia article, two questions on Academia.SE, and a hilarious thread on scienceblogs with passages like "I’m we-ing all over myself", "it is apparently meant to include the author and the reader, and presupposes that the reader always agrees with anything the author might say", and "As kids we were schooled to avoid ‘I’, lest we be look like we had big egos. Instead we should be humble, and therefore all first person was the plural. This was taught to us by teachers who thought their own opinions were pure gold and more important than anything else on earth".
I guess this tells us why the practice persists, but not how it originated. The best I could find is Grammarphobia blog quoting Oxford English Dictionary, whose earliest example of nosism is "Old English translation of a 5th-century Christian history written in Latin by Paulus Orosius, a student of St. Augustine". We doubt however that scientists of all people would have been swayed by an obscure student of St. Augustine. More importantly, at least in the English translation, Ptolemy is already we-ing in the Almagest three centuries earlier. Of course, we don't know if we can trust that modern translators did not insert modern usage, but they did translate Archimedes and Apollonius referring to themselves as "I".
So where did nosism come from and how did it take hold? Is Ptolemy we-ing in the original, and was he the one who started the practice in science (that would make more sense than Orosius)? Or was it somebody else?