I don't know about it, but I guess it's a kind of 当て字 (a phonetic equivalent; a substitute character using kanji). People here started to speak Japanese first, and later Kanji (Chinese characters) were imported for writing. 当て字 are phonetic equivalent, but have no meanings. It's just my guess.

http://gogen-allguide.com/ha/haniwa.html こんな説があるそうですよ。