Contender Advantage is an inherent trait in open debates, in which the contender has a higher chance in winning a debate. There are a number of reasons that the advantage shows up in DDO debate statistics, the most compelling being Last Word, Newb Sniping, and Choice of Acceptance.
The Last Word advantage applies to contenders in that in all DDO formats, the contender gets the opportunity to speak last. By speaking last, the instigator does not have an opportunity to refute anything said in that argument. Many try to counter this by saying that the final round is for summaries only, however others have countered that doing so would then give the Last Word advantage to the instigator, who already has the advantage of having the First Word and defining the resolution.
The only accurate way to remove a Last Word advantage would be for both instigator and contender to post all arguments at the same time (this would also remove the First Word advantage). However, this is not currently possible under DDO's debate coding. What is commonly done is that the contender is allowed to address all current arguments, but not allowed to make any more (doing so would cost the conduct point).
See the full article on Newb Sniping for more detail.
Newb Sniping can only be done as a contender, so the advantage of it directly translates into a contender advantage.
Choice of Acceptance
The choice of acceptance can best be summed up as contenders can choose to accept a debate or not, based on the topic, resolution, definition, and person that posted the challange. While a challenger has little control over who accepts their debate.