In a “material not included in the film” YouTube video, I saw a panel from a TFA comic book that had Chewbacca tearing off 3P0’s arm, reason unspecified. Could be unrelated, but I find it likely that it is related to some deleted scene or unfilmed part of the script or something.
I don’t know what the actual deal with this was in the script and with the deleted scenes, but I found it interesting that at the end of the film you can see C-3P0 waving with his newly restored golden left arm, and just a few feet away from him is another protocol droid who’s whole body is the same shade of red as 3P0’s arm in this film.