After post has been out for a while there probably is a majority in support. However every single time I read the comments of these kind of threads the first several highly upvoted comments are almost always anti-union. Its not until the post has been on the front page for a while before the comments begin to turn.
Do I have sciencily evidence? Nope. Could it be a confirmation bias or just a coincidence? Sure. However it seems to happen a lot.
You may be running into the contrarian dynamic: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor.... If an article is about unions, the first round of objections is probably going to come from the "boo unions" contingent. Once those comments appear, a second wave of objections saying "wait a minute, unions have done good things" (or "I can't believe how anti-union these comments are!") would be expected.
I don't know for sure if that's what's happening in the threads you're talking about but it kinda sounds like it might be.
Do I have sciencily evidence? Nope. Could it be a confirmation bias or just a coincidence? Sure. However it seems to happen a lot.