The yuppie mystery is solved. It appears that all the early 20th century occurrences are spurious. I did a search for the word in Google books and that threw up 21 documents. Of these, one was wrongly dated 1913. It was in fact published in the 1980s. The rest seem to be the result of faulty scanning or faulty OCR. For instance, one book contained the phrase APPLE BLOSSOM in a strange font. The first word was interpreted as YUPPIE. In another book the word supplement was interpreted. as yuppie -ment.
Phooey!
The moral of this sorry tale is be very suspicious of ngram blips.
This should teach corpus linguists a lesson!