I'm the site owner. It's open again, temporarily. Somebody's script has multiple machines (many are EC2 instances) downloading the same file many times per second. That's not something I want to encourage.
Interesting. I suppose an automated DDoS of everything on the hackernews front page isn't impossible.
Are there other explanations? Scribd definitely got it automatically at least once, maybe they do it too many times? How many HN users use EC2-based proxies?