Is there even a makor for back peyos?
Yes. Its called both pikuach nefesh and practicality. Sadly, hiding peyot behind the ears and/or wearing kipot toward the back of the head was likely deemed an inyan of pikuach nefesh at different times and places, so one would not be recognized as a Jew during times of anti-semitic violence. Clearly, that may be a legit concern in parts of Europe verus Bnai Brak. Also, for some who work in certain professions, I supsect having your peyos behind the ear may simply be a more practical option.