My young son asked me this question, but I can't deliver an answer, except to make broad guesses. Does anyone know for sure?
Since the above etymology indicates that ass in the sense of buttocks comes from arse, here is the etymology of arse:
arse
Who said it comes from ears?
Glenn said
Who said it comes from ears?
Random House did, for ass(2) (meaning butt, asshole, or sex actl)
I don't have the Random House, but I would double check if they are referring to the Old English word for ass, which happens to be spelled ears or if they are providing a Modern English translation of some Old English word which somehow meant ears.
Sometimes there are subtle differences in punctuation or font that make big differences in how you understand the etymology as noted in a dictionary.
I strongly suspect ears in this etymology is the spelling of an Old English word for ass and has nothing to do with modern ears.