I think it's someone bumping the thread. Before the name and domain change they removed the bump button but after the shift I think they didn't remember to use the one with no bump button.
Sort of. The bump button is there on purpose. We removed it for a bit, but that just led to an enormous flood of useless messages bumping threads. People would create new accounts just to post "up" or "bump" so even using the warn or ban options didn't really slow it down. I get the gripes about the bump button, but it's still a better option than getting a reply notification and the thread getting bumped anyway.