It refers to evaporative cooling of group belief - https://www.lesswrong.com/posts/ZQG9cwKbct2LtmL3p/evaporativ...

In evaporative cooling, the warmer particles are more prone to evaporate and escape leaving the remaining substance cooler than it was. Likewise, as individuals with more of a libertarian or founder-sympathizing mindset stopped contributing to HN as much, the overall tone of the site turned more negative against business and successful founders.[1] I don't want to speculate too much on why they started leaving--I hope it's because they all got too busy being successful founders--but once the effect started to happen and HN turned into less and less of a site for founders to discuss their startups and turned more and more into the proverbial peanut gallery, I think a feedback loop started pushing more of them away.

mftrhu is probably correct in attributing the origin of the analogy but I don't specifically remember where I got it from and I think some of the examples EY gives in that essay would be uncalled for to apply as a direct analogy to HN. HN was never a doomsday cult and I'm not even trying to say that all of the smart and reasonable people left HN, but rather that there was a specific attitude and mentality that's not well represented here anymore.

[1] This is oversimplified. Classic HN still had lots of people complaining about big tech companies, it's just that the criticism was usually voiced from the perspective of another founder rather than from the perspective of a progressive critic. For instance, I remember lots of complaints about Apple's arbitrary and capricious App Store policies, but then that directly affects the startup founder who wants to build an iPhone app.