The article here takes a bit stronger stance than "losing debates because of tweets":
The NSDA has allowed hundreds of judges with explicit left-wing bias to infiltrate the organization. These judges proudly display their ideological leanings in statements—or “paradigms”—on a public database maintained by the NSDA called Tabroom, where they declare that debaters who argue in favor of capitalism, or Israel, or the police, will lose the rounds they’re judging.
The article calls out five judges for being biased. The NSDA site shows 47,168 paradigms. So, while there may be an issue, there doesn't seem to be much proof here. It could equally well be that the author is cherry-picking instances that fit his ideology.