The deer are getting smart.
I have heard of this around here. Deer go rummaging through you trash, find your old 80's era jogging outfits, as well as tossed Halloween costumes, and dress up like humans.
Really, what you saw was a massive buck, and he didnt want to take any chances. See, his tail was still most likely exposed, and by hiding behind a tree, all you could see what the mask and the bright parts of his outfit.
Man, these deer, are getting really smart.