Because he can? I'm not speaking on his behalf but I don't think he exactly owes you an explanation for that question. I used to trust a person somewhat and I don't feel like trusting him anymore, I can always change my feedback from neutral to negative? It's completely fine to do that provided a valid reason is tagged along with it.(which seems to be the case here)
Had that been the case, I don't think actmyname and DarkStar_ would have countered it(,unless you think actmyname and DarkStar_ are Anduck's alts and actmyname and DarkStar_ are also chipmixer shills, well....Digaran......) But that wasn't the case, read this:
Also reminding you of this, as some people seem to skip over it: Vod asked me to change my rating, threatening to otherwise red-rate me. I refused. He rated me red. He also started claiming I'm a scammer and a liar, when I'm provably neither. His ratings to me are absolutely dishonest -- not just "poor". Why are some people still thinking his ratings have something to do with some >2yr old auction, that Vod earlier commented by saying "I decided what you did wasn't untrustworthy"?