A DDOS of HN is quite strange indeed. There should be some kind of ransom involved, but seeing as HN doesn't make money (I assume) from these forums, there is nothing to ransom.
Why would it have to be for gain? I bet there's plenty of bored people with idle small- to medium-sized botnets who'd take down a somewhat popular site just for lulz.
For the people doing it, the gain can be about trying out their skills, not being bored, and getting the bragging rights of having DDoSed "Hacker" News.
Sure. I didn't want to put a how-to manual in this thread on the day of the attack though.
HN has a big table of closures representing actions a user can take. Loading certain pages, such as the reply link on a comment creates more of them. They time out after a while, which will get you "unknown or expired link". Intentionally creating a few million of them ought to fill up the server's memory and would be a more effective way to to impair the server's functionality than simply requesting the home page a few billion times.