I’m pretty sure it’s related to how the default sorting algorithm works, the default sort typically being Active. If the description in the docs is still accurate, I think maybe there’s something a little off in the decay time*, making it so posts linger, people keep commenting & conversing in comments, which keeps posts propped up, until finally at some point activity subsides enough for it to allow other posts to show up.

*-(possibly taking too long to take effect or not being weighted heavily enough?)