lol I do not know what causes this, but I have seen it before. It ''seems'' to happen in an older doe and ''seems'' to be in more of the pets/docile. I've seen it 3 times in 14 years. Each time it showed up in the deer's later years, and each time it was a docile/pet deer. I haven't a clue what causes it, hopefully somebody on here does. It never negatively affected them, just made them look kinda ''stupid'' or something. lol