You can find more info here: http://www.rimmkaufman.com/rkgblog/2010/09/28/negative-keywo...
Do you maintain "scent" from keyword to ad copy for this keyword? http://www.grokdotcom.com/topics/senseofscent.htm
If scent is not maintained you are hurting your clickthrough rate which accounts for a majority of the weighting behind quality score.