Believe me, it's practically the same conversations on HN as a bunch of guys at the bar arguing over who the best player is in the NFL.
Nerds are generally proud of their mental dexterity and cunningness, so this is what they feel defines them. In regular life, they generally are the smartest in a room, so overtime they learn to teach themselves: well hey, I might not be the coolest or X, but everyone knows I'm really smart so I'm gonna be that. They learn how to be the "alpha intelligence" so to speak, and quickly dismiss foolish ideas. This ensures their status as alpha and also increases their pride. How could you believe that?! they think to themselves and chuckle with a hint (or more) of schadenfreude.
Now they bring all this behavior to HN, but HN has some pretty smart people so they are a bit more tame. However, you'll always see in "New Product Released" something along these lines: "Well sure, it's great, but X, Y, Z." This is them just doing what they know how to do best -- poke holes. Also, this is HN and not only is it nerds, but hackers. Hackers are fully trained and employed at being master logicians. We are paid at work to poke holes. After all, it's a real waste of time to code it X way when you shoulda just had the foresight to know Y was going to be a problem!
To try and roughly summarize these rough thoughts:
-- Nerds link their intelligence to their identity more strongly than others
-- Nerds enjoy this kind of conversation because it's stimulating. It's the thrill of the battle especially when one wins it.
-- Nerds might spend too much time in this mode of conversation and just learn to talk that way. Some are really not trying to be jerks at all, but just feel that's the way you talk.