I don't think this phenomenon is just on Hacker News. I believe the recent negativity shift is an actual change in the startup community's perceptions of the world.
When things are "hot" in our industry, and seemingly trivial companies are exiting for billions, the people who are still struggling start to develop a somewhat bad attitude. I see it all the time particularly from people who are working on "serious stuff" and then lash out when they see Instagram's success (which they've been saying for years: "sure, but what's their revenue model??").
I don't condone it and honestly think it's genuinely strange. Now is as amazing a time as any to be founding a startup. Success of others (no matter how trivial) only makes things better.