"Popcorn Posting" is defined as the act of adding a comment to a thread, not to contribute to the discussion, but just to laugh at the subject matter or comment on the behavior of other tropers. The name is taken from our trope Pass the Popcorn.
Posts like these derail threads and encourage others to start conversing along with you, turning what was probably meant to be a serious discussion into a peanut gallery. It breaks the cardinal rule of the site, which is Don't Be A Dick, and makes it more likely for others to start being dicks, as well.
It can happen on ATT, when people post on threads just to joke about the person being reported, even after the mods have already stepped in (a phenomenon also known as "Gravedancing"). It can happen on TLP, where underwritten drafts get dogpiled and end up with unproductive snark comments long after the draft could've been discarded. It can happen on the forums, where thread-lurkers just can't resist to make comments about drama that they weren't involved in. Anywhere where posts and drama can happen, "Popcorn Posts" can happen.
We get it; drama can be fun and we all want a say. But "Popcorn Posting" isn't okay to do, not even if everyone else is doing it, not even if you're poking fun at a particularly unsubtle ban-evader, not even if you're just really dying to make that one joke. Our forums are not an Ochlocracy. Once or twice isn't the end of the world, but do this repeatedly enough and you can earn yourself a suspension.
If you see other people "Popcorn Posting", don't engage. On ATT, you can request the thread to be mod-locked to prevent future conversing. On TLP, you can get the thread discarded early by contacting a mod. Unproductive forum posts can be hollered and likely thumped by the mods, preventing further derails.