Defensiveness? Possibly, but that's just one. I believe they do it mostly because they want to antagonize. They don't care if the info is false because it's all just a game anyway. If there's fact-checking, it proves that the information is valuable enough to cause someone of importance to waste time debunking it. Therefore, it is high-value material and should go viral even more.

It's the same reason my dog steals socks. If someone is chasing them for a stolen sock, it must be of high value and therefore it must be stolen more frequently, carried away, and buried in the backyard. It's a part of the brain we share with dogs, apes, sheep, and cattle.