I don't think it's really about either, specifically. It's just my perspective, but maybe it symbolizes with the bow tie how women are breaking free of the gender-nonconformity of being 'feminine', and the 'nipple slip', which most people would look at first or focus on, symbolizes how a woman's body is constantly being sexualized when it's really just another nipple.
Or it could just be that breasts are beautiful and bow ties are awesome.
That definitely is a possibility. I do tend to see, or at least have seen (I have pretty much abandoned dA except for the occasional check-in), that there are usually DDs that have some sort of nudity. Though of course, a correctly taken photograph of a real artistic nude can be very beautiful and deserving of a DD.