It's Digg's 2nd birthday and in the article announcing that the comments are following a new tradition. Check it out:
http://digg.com/tech_news/Two_Years_of_Digging
(submitted by Kevin Rose)

OK, why is practically every comment on this thread dugg down to invisibility?

I think it's all done by the same person..
Not the first time it happened, one of the commenters said.

Ya, I've seen it happen before.