Ok, there was another factor. When a site is the source of many stories marked lightweight, it eventually gets penalized as a lightweight site. We have software that does this, plus moderators do it.
This was the case here: that site has been the source of, not spam exactly (which is why it isn't banned), but a lot of unsubstantive and/or derivative articles. The post you submitted is an example of the latter, since it was derived from a Reddit thread.
The penalties I'm talking about don't make it impossible for a story to get traction, but they do set the bar higher. So you were right that it was randomness, but the randomness was also skewed.